Optical recording/reproducing apparatus including a mask device for masking marginal rays, in a direction perpendicular to a recording medium track, of a light beam returned from the recording medium

ABSTRACT

An optical recording/reproducing apparatus includes an irradiation optical system for irradiating a light beam from a light source onto a predetermined track of an optical recording medium having a plurality of neighboring tracks as a fine light spot so as to perform recording/reproduction of information or reproduction of information, and a detection optical system for detecting a returned light beam from the optical recording medium. A mask is arranged in a far field region sufficiently separated from a focal plane of the detection optical system, for masking marginal rays, in a direction perpendicular to the track, of the returned light beam, so that information reproduced from a track adjacent to the predetermined track upon reproduction of information on the predetermined track is reduced.

This application is a continuation of application Ser. No. 08/351,511,filed Dec. 7, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical recording/reproductionapparatus for recording/reproducing information on/from or reproducinginformation from an optical disc, a magneto-optical disc, or the likeand, more particularly, to an optical arrangement which reducescrosstalk from an adjacent track and intersymbol interaction from anadjacent mark by inserting an aperture in the pupil or the far fieldregion of a detection optical system for detecting a returned light beamfrom an optical disc so as to prevent marginal rays from becomingincident on a photodetector.

2. Related Background Art

In recent years, various optical memories which performrecording/reproduction using a semiconductor laser beam have been placedon the market, and in particular, magneto-optical recording/reproductionapparatuses which can rewrite information are considered to bepromising.

A magneto-optical recording/reproduction apparatus magnetically recordsinformation by utilizing a local temperature rise of a magnetic thinfilm upon spot irradiation of a laser beam, and reproduces informationby a magneto-optical effect (Kerr effect). Recently, as the informationamount to be processed by, e.g., a computer has increased, a furtherincrease in density of the magneto-optical recording/reproductionapparatus has been studied. In order to increase the density, theinterval between two adjacent recording tracks is decreased, the lengthof each recorded mark is shortened, or mark edge recording is adopted inplace of mark position recording. On the other hand, in an optical headas well, the wavelength of a laser beam is reduced to decrease the lightspot diameter, or super-resolution is used so as to increase thedensity.

An optical system of an optical head using the super-resolution in aconventional optical recording/reproduction apparatus will be describedbelow.

An example of application of super-resolution to a light spot on anoptical disc using an annular aperture described in Japanese Laid-OpenPatent Application No. 56-116004 will be described below with referenceto FIG. 1. A light beam emitted from a semiconductor laser 1 iscollimated by a collimator lens 2, and is then incident on a beamsplitter 3. The light beam transmitted through the beam splitter 3 formsa fine light spot 9 on a magneto-optical disc 5 by an objective lens 4via an annular aperture 8. The annular aperture 8 is constituted byarranging a circular mask (diameter ε) at the central portion of anormal circular aperture (diameter A). Returned light from themagneto-optical disc 5 is reflected by the beam splitter 3 via theobjective lens 4, and is guided to a photodetector 7 by a condenser lens6.

FIG. 2 shows the sectional shape of the light spot 9 obtained when theannular aperture is used. In FIG. 2, the abscissa represents a value inunits of NA/λ where λ is the wavelength of a light beam from thesemiconductor laser 1, and NA is the numerical aperture of the objectivelens 4. The ordinate represents a value normalized with the centralintensity of the light spot 9. For the sake of simplicity, a case willbe examined below wherein a light beam having an almost uniformintensity distribution is incident on the objective lens 4. A curve acorresponds to a circular aperture, and a curve b corresponds to theannular aperture having ε=0.5 A. If the light spot diameter is definedby an Airy disk, the light spot diameter obtained when an annularaperture is used decreases to about 82% with respect to that of acircular aperture. The resolution of the optical system can be improvedaccordingly.

However, the intensity of a side lobe of the light spot is as low as 2%or less of the central intensity when a circular aperture is used, whilethe intensity of the side lobe is as high as 10% when an annularaperture is used. For this reason, the crosstalk amount from an adjacenttrack and the intersymbol interaction amount from an adjacent markundesirably increase. In a magneto-optical disc apparatus which requireshigh power upon recording/erasing of information, light utilizationefficiency is considerably lowered when an annular aperture is utilized.

An example of application of the super-resolution to a light spot on aphotodetector using a pinhole described in Japanese Laid-Open PatentApplication No. 2-168439 will be described below with reference to FIG.3. The same reference numerals in FIG. 3 denote optical parts having thesame functions as in FIG. 1.

A light beam emitted from a semiconductor laser 1 is collimated by acollimator lens 2, and is incident on a beam splitter 3. The light beamtransmitted through the beam splitter 3 forms a fine light spot 9 on amagneto-optical disc 5 by an objective lens 4. Returned light from themagneto-optical disc 5 is reflected by the beam splitter 3 via theobjective lens 4, and is guided to a photodetector 7 by a condenser lens6. A pinhole 11 is arranged at the focal point position of the condenserlens 6 in front of the photodetector 7.

FIG. 4 shows the shape of a light spot 10 at the focal point position inan enlarged scale. The light spot 10 has a similar shape to that shownin FIG. 2. The pinhole 11 allows only the central portion of the lightspot to pass therethrough, and guides it to the photodetector 7, thusmasking the side lobe. For this reason, crosstalk components from anadjacent track and intersymbol interaction components from an adjacentmark in the track direction included in the side lobe portion can beremoved.

For example, if the focal lengths of the objective lens and thecondenser lens are respectively set to be fo=3 mm and fc=30 mm, the NAof the objective lens is set to be 0.55, and the wavelength of thesemiconductor laser is set to be λ=780 nm, the spot diameter (defined by1/e² of the central intensity) of the light spot 10 is about 12 μm, andthe diameter of the pinhole for masking the side lobe must be about 15μm. Therefore, it becomes very difficult to align the light spot and thepinhole in both the optical axis direction and a planar directionperpendicular thereto. In the optical axis direction, the depth of thefocus of the light spot 10 is about 140 μm. When the light spot 9 on thedisc suffers a defocus of 1 μm, the focal point position of the lightspot 10 is shifted by 200 μm corresponding to a value twice thelongitudinal magnification, and falls outside the depth of the focus atthe pinhole position. As a result, the side lobe cannot be effectivelymasked. In the planar direction perpendicular to the optical axis aswell, when a light beam incident on the condenser lens is tilted by only1' due to a change in temperature or aging, a half of the light spot 10is undesirably masked by the pinhole 11, and as a result, signalreproduction is disabled.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of the abovesituation, and has as its object to provide an opticalrecording/reproduction apparatus capable of high-density recording,which can effectively remove crosstalk components from an adjacent trackand intersymbol interaction components from an adjacent mark in thetrack direction, which are included in a side lobe portion, can simplifythe arrangement and adjustment, and is stable against a change intemperature and aging.

In order to achieve the above object, according to the presentinvention, an optical recording/reproduction apparatus is arranged asfollows.

According to the present invention, when recording/reproduction ofinformation or reproduction of information is performed by irradiating alight beam emitted from a light source onto a predetermined track on anoptical recording medium having a plurality of neighboring tracks as afine light spot, mask means for masking marginal rays, in a directionperpendicular to the track, of the returned light beam from the opticalrecording medium is arranged in a far field region sufficientlyseparated from the focal plane of a detection optical system fordetecting a returned light beam from the optical recording medium in anoptical path of the detection optical system, thereby reducinginformation (crosstalk from an adjacent track) reproduced from a trackadjacent to the predetermined track upon reproduction of informationfrom the predetermined track.

Also, according to the present invention, when recording/reproduction ofinformation or reproduction of information is performed by irradiating alight beam emitted from a light source onto a predetermined track on anoptical recording medium having a plurality of neighboring tracks as afine light spot, mask means for masking marginal rays, in the trackdirection, of the returned light beam from the optical recording mediumis arranged in a far field region sufficiently separated from the focalplane of a detection optical system for detecting a returned light beamfrom the optical recording medium in an optical path of the detectionoptical system, thereby reducing information (intersymbol interaction)reproduced from a mark adjacent to a predetermined mark on a given trackupon reproduction of the predetermined mark on the given track.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information;

a detection optical system for detecting a returned light beam from theoptical recording medium; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in a directionperpendicular to the track, of the returned light beam,

wherein by arranging the mask means, information reproduced from a trackadjacent to the predetermined track upon reproduction of information onthe predetermined track is reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information bytime-serially scanning a plurality of marks located on the predeterminedtrack;

a detection optical system for detecting a returned light beam from theoptical recording medium; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in a trackdirection, of the returned light beam,

wherein when the light spot reproduces a predetermined mark located onthe predetermined track, information reproduced by the light spot from amark adjacent to the predetermined mark on the predetermined track isreduced by arranging the mask means.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information;

a detection optical system for detecting a returned light beam from theoptical recording medium, the detection optical system including a firstphotodetector for detecting a reproduction signal of information and asecond photodetector for detecting a position signal in a directionperpendicular to the track on a surface of the optical recording medium,and the first and second photodetectors being constituted by a singlephotodetector; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking margin rays, in the directionperpendicular to the track, of the returned light beam,

wherein the mask means satisfies the following relation:

    0.74-0.21·(d1/p)<B1/A1<1.09-0.21·(d1/p) for 0<B1/A1<1

where

A1: the beam diameter of the returned light beam in the directionperpendicular to the track

B1: the aperture width of the mask means for masking the returned lightbeam in the direction perpendicular to the track

d1: the 1/e² diameter of the light spot on the optical recording mediumin the direction perpendicular to the track

p: the track pitch of the optical recording medium, whereby informationreproduced by a side lobe of the light spot from a track adjacent to thepredetermined track upon reproduction of information of thepredetermined track by the light spot is reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information;

a detection optical system for detecting a returned light beam from theoptical recording medium, the detection optical system including a firstphotodetector for detecting a reproduction signal of information and asecond photodetector for detecting a position signal in a directionperpendicular to the track on a surface of the optical recording medium,and the first and second photodetectors being constituted by differentphotodetectors; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in the directionperpendicular to the track, of the returned light beam,

wherein the mask means satisfies the following relation:

    0.64-0.21·(d1/p)<B1/A1<1.09-0.21·(d1/p) for 0<B1/A1<1

where

A1: the beam diameter of the returned light beam in the directionperpendicular to the track

B1: the aperture width of the mask means for masking the returned lightbeam in the direction perpendicular to the track

d1: the 1/e² diameter of the light spot on the optical recording mediumin the direction perpendicular to the track

p: the track pitch of the optical recording medium,

whereby information reproduced by a side lobe of the light spot from atrack adjacent to the predetermined track upon reproduction ofinformation of the predetermined track by the light spot is reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information;

a detection optical system for detecting a returned light beam from theoptical recording medium, the detection optical system including a firstphotodetector for detecting a reproduction signal of information and asecond photodetector for detecting a position signal in a directionperpendicular to the track on a surface of the optical recording medium,and the first and second photodetectors being constituted by a singlephotodetector; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in the directionperpendicular to the track, of the returned light beam,

wherein a dominant optical aberration of the light spot is a coma, and

the mask means satisfies the following relation:

    0.74-0.21·(d1/p)-0.25·W31<B1/A1<1.09-0.21·(d1/p)-0.25·W31 for 0<B1/A1<1

where

A1: the beam diameter of the returned light beam in the directionperpendicular to the track

B1: the aperture width of the mask means for masking the returned lightbeam in the direction perpendicular to the track

d1: the 1/e² diameter of the light spot on the optical recording mediumin the direction perpendicular to the track

p: the track pitch of the optical recording medium

W31: the wave aberration coefficient of the coma,

whereby information reproduced by a side lobe, generated by the coma, ofthe light spot from a track adjacent to the predetermined track by thelight spot is reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information;

a detection optical system for detecting a returned light beam from theoptical recording medium, the detection optical system including a firstphotodetector for detecting a reproduction signal of information and asecond photodetector for detecting a position signal in a directionperpendicular to the track on a surface of the optical recording medium,and the first and second photodetectors being constituted by differentphotodetectors; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in the directionperpendicular to the track, of the returned light beam,

wherein a dominant optical aberration of the light spot is a coma, and

the mask means satisfies the following relation:

    0.64-0.21·(d1/p)-0.25·W31<B1/A1<1.09-0.21·(d1/p)-0.25·W31 for 0<B1/A1<2

where

A1: the beam diameter of the returned light beam in the directionperpendicular to the track

B1: the aperture width of the mask means for masking the returned lightbeam in the direction perpendicular to the track

d1: the 1/e² diameter of the light spot on the optical recording mediumin the direction perpendicular to the track

p: the track pitch of the optical recording medium

W31: the wave aberration coefficient of the coma, whereby informationreproduced by a side lobe, generated by the coma, of the light spot froma track adjacent to the predetermined track upon reproduction ofinformation of the predetermined track by the light spot is reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information;

a detection optical system for detecting a returned light beam from theoptical recording medium, the detection optical system including a firstphotodetector for detecting a reproduction signal of information and asecond photodetector for detecting a position signal in a directionperpendicular to the track on a surface of the optical recording medium,and the first and second photodetectors being constituted by a singlephotodetector; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in the directionperpendicular to the track, of the returned light beam,

wherein a dominant optical aberration of the light spot is a sphericalaberration, and

the mask means satisfies the following relation:

    0.74-0.21·(d1/p)-0.26·W40.sup.2 <B1/A1<1.09-0.21·(d1/p)-0.26·W40.sup.2 for 0<B1/A1<1

where

A1: the beam diameter of the returned light beam in the directionperpendicular to the track

B1: the aperture width of the mask means for masking the returned lightbeam in the direction perpendicular to the track

d1: the 1/e² diameter of the light spot on the optical recording mediumin the direction perpendicular to the track

p: the track pitch of the optical recording medium

W40: the wave aberration coefficient of the spherical aberration,

whereby information reproduced by a side lobe, generated by thespherical aberration, of the light spot from a track adjacent to thepredetermined track upon reproduction of information of thepredetermined track by the light spot is reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information;

a detection optical system for detecting a returned light beam from theoptical recording medium, the detection optical system including a firstphotodetector for detecting a reproduction signal of information and asecond photodetector for detecting a position signal in a directionperpendicular to the track on a surface of the optical recording medium,and the first and second photodetectors being constituted by differentphotodetectors; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in the directionperpendicular to the track, of the returned light beam,

wherein a dominant optical aberration of the light spot is a sphericalaberration, and

the mask means satisfies the following relation:

    0.64-0.21·(d1/p)-0.26·W40.sup.2 <B1/A1<1.09-0.21·(d1/p)-0.26·W40.sup.2 for 0<B1/A1<1

where

A1: the beam diameter of the returned light beam in the directionperpendicular to the track

B1: the aperture width of the mask means for masking the returned lightbeam in the direction perpendicular to the track

d1: the 1/e² diameter of the light spot on the optical recording mediumin the direction perpendicular to the track

p: the track pitch of the optical recording medium

W40: the wave aberration coefficient of the spherical aberration,

whereby information reproduced by a side lobe, generated by thespherical aberration, of the light spot from a track adjacent to thepredetermined track upon reproduction of information of thepredetermined track by the light spot is reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information bytime-serially scanning a plurality of marks located on the predeterminedtrack;

a detection optical system for detecting a returned light beam from theoptical recording medium; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in the trackdirection, of the returned light beam,

wherein the mask means satisfies the following relation:

    0.77-0.1·(d2/m)<B2/A2<1.07-0.1·(d2/m) for 0<B2/A2<1

where

A2: the beam diameter of the returned light beam in the track direction

B2: the aperture width of the mask means for masking the returned lightbeam in the track direction

d2: the 1/e² diameter of the light spot on the optical recording mediumin the track direction

m: the shortest mark length on the optical recording medium,

whereby information reproduced by the light spot from a mark adjacent toa predetermined mark on the predetermined track upon reproduction of thepredetermined mark located on the predetermined track by the light spotis reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information bytime-serially scanning a plurality of marks located on the predeterminedtrack;

a detection optical system for detecting a returned light beam from theoptical recording medium; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in the trackdirection, of the returned light beam,

wherein a dominant optical aberration of the light spot is a coma, and

the mask means satisfies the following relation:

    0.77-0.1·(d2/m)-0.12·W31<B2/A2<1.07-0.1·(d2/m)-0.12·W31 for 0<B2/A2<1

where

A2: the beam diameter of the returned light beam in the track direction

B2: the aperture width of the mask means for masking the returned lightbeam in the track direction

d2: the 1/e² diameter of the light spot on the optical recording mediumin the track direction

m: the shortest mark length on the optical recording medium

W31: the wave aberration coefficient of the coma, whereby informationreproduced by a side lobe, generated by the coma, of the light spot froma mark adjacent to a predetermined mark on the predetermined track uponreproduction of the predetermined mark located on the predeterminedtrack by the light spot is reduced.

An optical recording/reproduction apparatus according to the presentinvention comprises:

an irradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a fine light spot so as to performrecording/reproduction of information or reproduction of information bytime-serially scanning a plurality of marks located on the predeterminedtrack;

a detection optical system for detecting a returned light beam from theoptical recording medium; and

mask means, arranged in a far field region sufficiently separated from afocal plane of the detection optical system in an optical path of thedetection optical system, for masking marginal rays, in the trackdirection, of the returned light beam,

wherein a dominant optical aberration of the light spot is a sphericalaberration, and

the mask means satisfies the following relation:

    0.77-0.1·(d2/m)-0.12·W40.sup.2 <B2/A2<1.07-0.1·(d2/m)-0.12·W40.sup.2 for 0<B2/A2<1

where

A2: the beam diameter of the returned light beam in the track direction

B2: the aperture width of the mask means for masking the returned lightbeam in the track direction

d2: the 1/e² diameter of the light spot on the optical recording mediumin the track direction

m: the shortest mark length on the optical recording medium

W40: the wave aberration coefficient of the spherical aberration,

whereby information reproduced by a side lobe, generated by thespherical aberration, of the light spot from a mark adjacent to apredetermined mark on the predetermined track upon reproduction of thepredetermined mark located on the predetermined track by the light spotis reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view for explaining a conventional optical system;

FIG. 2 is a graph for explaining a light spot on an optical disc in theprior art shown in FIG. 1;

FIG. 3 is a view for explaining another conventional optical system;

FIG. 4 is a view for explaining a light spot on a photodetector in theprior art shown in FIG. 3;

FIG. 5 is a view for explaining the principle of the invention accordingto the first embodiment of the present invention;

FIG. 6 is a view for explaining the principle of the invention accordingto the first embodiment of the present invention;

FIG. 7 is a front view for explaining an aperture according to the firstembodiment of the present invention;

FIG. 8 is a graph for explaining the simulation results of the effect ofthe invention according to the first embodiment of the presentinvention;

FIG. 9 is a graph for explaining the experimental results according tothe first embodiment of the present invention;

FIG. 10 is a graph for explaining the simulation results of the effectof the invention according to the first embodiment of the presentinvention;

FIG. 11 is a view for explaining the principle of the inventionaccording to the first embodiment of the present invention;

FIG. 12 is a view for explaining the principle of the inventionaccording to the first embodiment of the present invention;

FIG. 13 is a graph for explaining the simulation results of the effectof the invention according to the first embodiment of the presentinvention;

FIG. 14 is a graph for explaining the simulation results of the effectof the invention according to the first embodiment of the presentinvention;

FIG. 15 is a view for explaining an optical system according to thefirst embodiment of the present invention;

FIG. 16 is a circuit diagram for explaining signal detection accordingto the first embodiment of the present invention;

FIG. 17 is a graph for explaining the simulation results of the effectof the invention according to the first embodiment of the presentinvention;

FIG. 18 is a graph for explaining the simulation results of the effectof the invention according to the first embodiment of the presentinvention;

FIG. 19 is a view for explaining an optical system according to thesecond embodiment of the present invention;

FIG. 20 is a circuit diagram for explaining signal detection accordingto the second embodiment of the present invention;

FIG. 21 is a front view for explaining an aperture according to thethird embodiment of the present invention;

FIG. 22 is a graph for explaining the simulation results of the effectof the invention according to the third embodiment of the presentinvention;

FIG. 23 is a view for explaining an optical system according to thefourth embodiment of the present invention;

FIG. 24 is a circuit diagram for explaining signal detection accordingto the fourth embodiment of the present invention;

FIG. 25 is a view for explaining the principle of the inventionaccording to the fifth embodiment of the present invention;

FIG. 26 is a view for explaining the principle of the inventionaccording to the fifth embodiment of the present invention;

FIG. 27 is a front view for explaining an aperture according to thefifth embodiment of the present invention;

FIG. 28 is a graph for explaining the difference between the prior artand the fifth embodiment of the present invention;

FIG. 29 is a view for explaining an optical system according to thefifth embodiment of the present invention;

FIG. 30 is a view for explaining another optical system according to thefifth embodiment of the present invention;

FIG. 31 is a view for explaining an optical system according to thesixth embodiment of the present invention;

FIG. 32 is a view for explaining an optical system according to thesixth embodiment of the present invention;

FIG. 33 is a circuit diagram for explaining signal detection accordingto the sixth embodiment of the present invention;

FIG. 34 is a view for explaining the principle of the inventionaccording to the seventh embodiment of the present invention;

FIG. 35 is a view for explaining the principle of the inventionaccording to the seventh embodiment of the present invention;

FIG. 36 is a front view for explaining an aperture according to theseventh embodiment of the present invention;

FIG. 37 is a graph for explaining the simulation results of the effectof the invention according to the seventh embodiment of the presentinvention;

FIG. 38 is a graph for explaining the experimental results according tothe seventh embodiment of the present invention;

FIG. 39 is a view for explaining an optical system according to theseventh embodiment of the present invention;

FIG. 40 is a circuit diagram for explaining signal detection accordingto the seventh embodiment of the present invention;

FIG. 41 is a view for explaining the principle of the inventionaccording to the eighth embodiment of the present invention;

FIG. 42 is a view for explaining the principle of the inventionaccording to the eighth embodiment of the present invention;

FIG. 43 is a front view for explaining an aperture according to theeighth embodiment of the present invention;

FIG. 44 is a view for explaining an optical system according to theeighth embodiment of the present invention;

FIG. 45 is a circuit diagram for explaining signal detection accordingto the eighth embodiment of the present invention;

FIG. 46 is a view for explaining the principle of the inventionaccording to the ninth embodiment of the present invention;

FIG. 47 is a view for explaining the principle of the inventionaccording to the ninth embodiment of the present invention;

FIG. 48 is a graph for explaining the intensity distribution of a lightspot;

FIG. 49 is a graph for explaining the simulation results of the effectof the invention according to the ninth embodiment of the presentinvention;

FIG. 50 is a graph for explaining the simulation results of the effectof the invention according to the ninth embodiment of the presentinvention;

FIG. 51 is a graph for explaining the simulation results of the effectof the invention according to the ninth embodiment of the presentinvention;

FIG. 52 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 53 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 54 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 55 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 56 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 57 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 58 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 59 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 60 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 61 is a graph for explaining the simulation results of the effectof the invention according to an embodiment of the present invention;

FIG. 62 is a view for explaining the principle of the inventionaccording to the 10th embodiment of the present invention;

FIG. 63 is a view for explaining the principle of the inventionaccording to the 10th embodiment of the present invention;

FIG. 64 is a front view for explaining an aperture according to the 10thembodiment of the present invention;

FIG. 65 is a graph for explaining the experimental results of the effectof the invention according to the 10th embodiment of the presentinvention;

FIG. 66 is a graph for explaining the experimental results of the effectof the invention according to the 10th embodiment of the presentinvention;

FIG. 67 is a graph for explaining the experimental results of the effectof the invention according to an embodiment of the present invention;

FIG. 68 is a graph for explaining the experimental results of the effectof the invention according to an embodiment of the present invention;

FIG. 69 is a front view for explaining the principle of the inventionaccording to the 11th embodiment of the present invention;

FIG. 70 is a front view for explaining the principle of the inventionaccording to the 11th embodiment of the present invention;

FIG. 71 is a graph for explaining the experimental results of the effectof the invention according to the 11th embodiment of the presentinvention; and

FIG. 72 is a graph for explaining the experimental results of the effectof the invention according to the 11th embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The arrangement of the first embodiment of an opticalrecording/reproduction apparatus according to the present invention willbe described below with reference to FIGS. 5 to 7. FIG. 5 is a sectionalview, in the radial direction, of a magneto-optical disc 5 in an opticalsystem of the present invention, FIG. 6 is a sectional view, in thetrack direction, of the disc 5, and FIG. 7 is a view showing the shapeof an aperture 12. FIGS. 5 and 6 show only a detection optical system(to be referred to as a light-receiving system hereinafter) fordetecting returned light from the magneto-optical disc 5 to explain theprinciple of the present invention.

Referring to FIG. 5, the magneto-optical disc 5 is tilted in the radialdirection of the magneto-optical disc 5, as indicated by an arrow 13,and a state wherein a side lobe due to a coma being generated in a lightspot 9 by the tilt of the disc 5 is illustrated beside thelight-receiving system. A light spot is illustrated in a state observedin the direction of its rays, and its a-a' section corresponds to theradial direction. When information is reproduced from a given track inthis state, a side lobe due to a coma undesirably reproduces informationof an adjacent track, and the reproduced information is found in areproduction signal as crosstalk components. In particular, in a discconsisting of a plastic material such as polycarbonate, a crosstalkproblem from an adjacent track due to a tilt of the disc cannot beavoided, and seriously disturbs an increase in density.

Returned light from the magneto-optical disc 5 is collimated via anobjective lens 4, and is guided toward a photodetector 7 by a condenserlens 6. An aperture 12 is arranged in the optical path between theobjective lens 4 and the condenser lens 6, i.e., in the vicinity of thepupil of the light-receiving system so that these rays do not reach thephotodetector 7. FIG. 5 illustrates rays which pass various heightpositions of the pupil diameter. As can be seen from FIG. 5, of theserays, hatched rays masked by the aperture 12 mainly form a side lobe ofa light spot on the disc. The aperture 12 is arranged in the far fieldregion sufficiently separated from the focal plane of thelight-receiving system and masks margin rays, in the radial directionperpendicular to the track, of a returned light beam.

Therefore, most of the crosstalk components from an adjacent track areincluded in the marginal rays, and if these rays are masked, thecrosstalk components can be reduced. The side lobe due to the coma andmarginal rays in the vicinity of the pupil of the light-receiving systemnormally have a good correspondence therebetween although they do nothave a strict one-to-one correspondence therebetween, unlike thatbetween the light spot on the disc and the light spot on the disc andthe light spot of the light-receiving system shown in FIG. 4.

Referring to FIG. 6, the magneto-optical disc 5 is not tilted in thetrack direction indicated by an arrow 14. Similarly, FIG. 6 illustrates,beside the light-receiving system, a state wherein a side lobe due to acoma is generated in the light spot 9. The b-b' section of the lightspot corresponds to the track direction. Returned light from themagneto-optical disc 5 is collimated via the objective lens 4, and isguided toward the photodetector 7 via the condenser lens 6. The aperture12 has a dimension in the track direction larger than a beam diameter Aso as not to mask marginal rays in the track direction. This is to guidemodulated components of a reproduction signal from the disc to thephotodetector as much as possible since these components are spatiallydistributed in the track direction in the pupil of the light-receivingsystem.

FIG. 7 is a front view of the aperture 12. The dimension, in the radialdirection, of the aperture is B1 (B1<A1), and the dimension, in thetrack direction perpendicular to the radial direction, of the apertureis C1 (C1>A1). If the objective lens 4 has a focal length fo=3 mm andNA=0.55, the beam diameter A1=3.3 mm. As will be described later, sincethe dimension B1 is selected to fall within a range of B1=1.3 to 2.8 mm,easy adjustment of the aperture is greatly improved as compared to theprior art in which a pinhole having a diameter of 15 μm is inserted inthe focal plane of the light-receiving system, and the aperture is noteasily influenced by a change in temperature or aging. Since theaperture 12 is inserted in the light-receiving system, a considerabledecrease in light utilization efficiency due to the insertion of anannular aperture can be prevented unlike in the prior art, and thisarrangement is suitable for, e.g., a magneto-optical discrecording/reproduction apparatus which requires high power uponrecording/erasing of information.

FIGS. 8 to 10 show the results of computer simulations and experimentsassociated with a crosstalk reduction effect from an adjacent track inthe optical system according to the first embodiment of the presentinvention.

FIG. 8 shows the calculation results of variations in crosstalk amountfrom an adjacent track and reproduction signal (carrier), which areobtained by varying a width B1, in the radial direction, of the aperture12. The wavelength of the semiconductor laser is λ=780 nm, the tilt ofthe disc is 5 mrad. in the radial direction, the NA of the objectivelens is NA=0.55, the track pitch is 1.4 μm, and the track width of therecorded portion is 0.9 μm. The mark length of the carrier is 0.75 μm,and the mark length of the crosstalk component recorded in the adjacenttrack is 3.0 μm. The abscissa represents the ratio of the width B1, inthe radial direction, of the aperture 12 with respect to the beamdiameter A1. The ordinate represents an amount normalized with thecarrier or the crosstalk amount from the adjacent track when no apertureis arranged (B1/A1=1). When B1/A1 is varied, the reduction effectappears when B1/A1=0.85 or less. For example, when B1/A1=0.7, thedecrease in carrier level is about 1 dB, while the decrease in crosstalkamount from the adjacent track is about 5 dB. Thus, a remarkable effectis confirmed. FIG. 8 also shows the crosstalk amount to the carrier,which is normalized with a value obtained without an aperture, and areduction effect of about 4 dB is confirmed.

FIG. 9 shows the experimental results of a variation in C/N (carrier tonoise) ratio of a reproduction signal (carrier), which are obtained byvarying the width B1, in the radial direction of the disc, of theaperture 12. The wavelength of the semiconductor laser is λ=780 nm, thetilt of the disc is 5 mrad. in the radial direction, the NA of theobjective lens is NA=0.55, the track pitch is 1.4 μm, and the trackwidth of the recorded portion is 0.9 μm. The mark length of the carrieris 0.75 μm, and the mark length of the crosstalk component recorded inthe adjacent track is 3.0 μm. The abscissa represents the ratio of thewidth B1, in the radial direction, of the aperture 12 with respect tothe beam diameter A1. The ordinate represents the C/N ratio of thereproduction signal. As B1/A1 is decreased, the carrier is lowered, andthe C/N ratio is lowered accordingly. In this case, the decrease in C/Nratio is smaller than that of the carrier level (see FIG. 8). Forexample, when B1/A1=0.7, the decrease in carrier level is about 1 dB,while the decrease in C/N ratio is about 0.5 dB. However, since thecrosstalk amount to the carrier is largely reduced as compared to a casewherein no aperture is arranged, stable signal reproduction is assuredas a whole.

FIG. 10 shows the calculation results of the crosstalk amount from anadjacent track to the carrier, which are obtained by varying the tilt ofthe disc. The parameter used is the ratio of the width B1, in the radialdirection, of the aperture 12 to the beam diameter A1. The abscissarepresents the crosstalk amount from an adjacent track to the carrier.As can be seen from FIG. 10, at any disc tilt, the reduction effectappears when B1/A1=0.85, the reduction effect is enhanced as the disctilt becomes small, and the reduction effect is gradually reduced as thedisc tilt becomes large. For example, when B1/A1=0.73 and the disc isnot tilted, a reduction effect of about 5 dB is expected.

Another effect of the first embodiment of the present invention will beexplained below with reference to FIGS. 11 and 12. FIG. 11 is asectional view, in the radial direction, of the magneto-optical disc 5in the optical system of the present invention, and FIG. 12 is asectional view, in the track direction, of the disc 5. FIGS. 11 and 12particularly illustrate only the light-receiving system for explainingthe principle of the present invention.

FIG. 11 also illustrates, beside the light-receiving system, a statewherein a side lobe due to a spherical aberration is generated in thelight spot 9. The spherical aberration is generated due to amanufacturing error of the objective lens and a substrate thicknesserror of the disc, and has a side lobe which is symmetrical about thecenter of rotation. A light spot is illustrated in a state observed inthe direction of its rays, and its a-a' section corresponds to theradial direction. When information on a given track is reproduced inthis state, the side lobe caused by the spherical aberration undesirablyreproduces information on an adjacent track, and the reproducedinformation is found in a reproduction signal as crosstalk components.When the NA of the objective lens is to be increased, the allowablemanufacturing error must be reduced, and the problem of crosstalk froman adjacent track due to a spherical aberration seriously disturbs anincrease in density.

Returned light from the magneto-optical disc 5 is collimated via theobjective lens 4, and is guided toward the photodetector 7 by thecondenser lens 6. The aperture 12 is arranged between the objective lens4 and the condenser lens 6, i.e., in the vicinity of the pupil of thelight-receiving system, and masks marginal rays in the radial direction,so that they do not reach the photodetector 7. FIG. 11 illustrates rayswhich pass various height positions of the pupil diameter. As can beseen from FIG. 11, of these rays, hatched rays masked by the aperture 12mainly form a side lobe of a light spot on the disc. Therefore, most ofthe crosstalk components from the adjacent track are included in themarginal rays, and the crosstalk can be reduced by masking these rays.The side lobe caused by the spherical aberration and the marginal raysin the vicinity of the pupil of the light-receiving system normally havea good correspondence therebetween as in the case of a coma.

Referring to FIG. 12, an arrow 14 and the b-b' section of a light spotcorrespond to the track direction perpendicular to the radial direction.Returned light from the magneto-optical disc 5 is collimated via theobjective lens 4, and is guided toward the photodetector 7 by thecondenser lens 6. The aperture 12 has a dimension, in the trackdirection, larger than the beam diameter A1 so as not to mask marginalrays in the track direction for the same reason as in FIG. 5.

FIGS. 13 and 14 show the computer simulation results of the reductioneffect of crosstalk components from an adjacent track in the opticalsystem according to the first embodiment of the present invention.

FIG. 13 shows the calculation results of the crosstalk from an adjacenttrack to the carrier, which are obtained when the width B1, in theradial direction, of the aperture 12 is set to be B1/A1=0.73 and variousspherical aberrations are generated. The spherical aberration (W40) wasvaried to ±0.53λ, ±0.40λ, ±0.27λ, and ±0.0λ. Other calculationconditions are the same as those in FIGS. 8 and 10. The abscissarepresents the defocus, and the ordinate represents the crosstalk amountfrom an adjacent track to the carrier. FIG. 14 shows similar calculationresults obtained when no aperture is arranged (B1/A1=1) and variousspherical aberrations are generated, for the sake of comparison.

As can be seen from a comparison between FIGS. 13 and 14, the crosstalkreduction effect can be obtained for each of the spherical aberrationsand defocus amounts. As can also be seen from FIGS. 13 and 14, as thespherical aberration becomes small, the reduction effect is enhanced; asthe spherical aberration becomes large, the effect is reduced. Forexample, when the spherical aberration=±0.27λ, a reduction effect ofabout 4 dB is obtained; when a spherical aberration=±0.53λ, a reductioneffect of about 2 dB is obtained.

FIGS. 15 and 16 show the entire magneto-optical head optical systemaccording to the first embodiment of the present invention. FIG. 15 is afront view of a magneto-optical disc recording/reproduction apparatus ofthe present invention, and FIG. 16 is a circuit diagram for explaining amethod of detecting a magneto-optical signal and servo signals. The samereference numerals in FIGS. 15 and 16 denote the same parts as in FIG.5, and a detailed description thereof will be omitted.

The magneto-optical head optical system shown in FIG. 15 is a so-calledseparated optical system. A light beam emitted from the semiconductorlaser 1 is collimated by the collimator lens 2. The light beam incidenton the polarization beam splitter 3 emerges from a stationary portionoptical system 29 toward an optical head movable portion 16, and formsthe fine light spot 9 on the magneto-optical disc 5 by the objectivelens 4. The optical head movable portion 16 carries the objective lens4, an actuator for driving the lens 4, a mirror 15, and the like. Inorder to shorten the access time to a predetermined track, the movableportion is constituted by the minimum number of required parts, and asemiconductor laser light source portion, a signal detection system, andthe like, which have a large weight, are arranged on the stationaryportion optical system 29.

The light beam, which is reflected by the recording medium surface andis incident again on the objective lens 4, is reflected by thepolarization beam splitter 3 via the mirror 15, and is guided toward asignal detection system. The light beam transmitted through a halfwaveplate 17 passes through the aperture 12 of the present invention, andsome marginal rays in the radial direction are masked. The aperture 12is arranged between the objective lens 4 and the condenser lens 6, i.e.,in the vicinity of the pupil of the light-receiving system, as describedabove. The light beam transmitted through the condenser lens 6 and acylindrical lens 19 is transmitted through or reflected by apolarization beam splitter 18, and is guided onto photodetectors 7-1 and7-2.

In an information reproduction mode, the semiconductor laser 1 emitslow-power light, and reproduction of a magneto-optical signal anddetection of servo signals are performed. In a recording mode, thesemiconductor laser 1 emits high-power light. Upon irradiation of ahigh-power laser beam, the temperature of the recording medium surfaceof the magneto-optical disc 5 rises, and the magnetization and thecoercive force are lowered. Therefore, when a magnetic head 50 applies amagnetic field whose polarity is inverted in correspondence withrecording information to the disc, a magneto-optical pit is recorded.

The detection system of a magneto-optical signal and servo signals willbe described below with reference to FIG. 16. FIG. 16 illustrates astate wherein the light beam reflected by the polarization beam splitter3 forms light spots 20-1 and 20-2 on the photodetectors 7-1 and 7-2 viathe condenser lens 6 and the cylindrical lens 19. In order to detect afocusing error signal by an astigmatism method, the photodetector 7-1 isarranged in the vicinity of the circle of least confusion, and a lightspot on the photodetector normally has a circular shape. In this case,however, since marginal rays in the radial direction are masked by theaperture 12, the light spot has a shape as shown in FIG. 16.

Sums of diagonal photoelectric conversion outputs of the light spot 20-1on the photodetector 7-1 are calculated, and a difference between thesums is differentially amplified by a differential amplifier 21 togenerate a focusing error signal 26. A difference, in the radialdirection, of the outputs of the light spot 20-2 on the photodetector7-2 is differentially amplified by a differential amplifier 22 togenerate a tracking error signal (push-pull signal) 27. Amagneto-optical signal 28 is detected in such a manner that the sumoutputs of the photodetectors 7-1 and 7-2 are generated by sumamplifiers 23 and 24, and thereafter, a difference therebetween isdifferentially amplified by a differential amplifier 25. Note that thephotodetector 7-1 can simultaneously obtain a tracking error signal fromthe differential output in the radial direction since it adopts aquadrant sensor.

Since the optical system shown in FIG. 15 adopts a single detectionsystem to detect the magneto-optical signal and the servo signals, somelight components, in the track direction, of a light beam are masked bythe aperture 12, and the amplitude of a tracking signal is undesirablylowered. FIGS. 17 and 18 show the computer simulation results of theinfluence of the aperture 12 on the tracking signal.

FIG. 17 shows the calculation results of a variation in amplitude of thetracking signal, which are obtained by varying the width B1, in theradial direction, of the aperture 12. The calculation conditions are thesame as those in FIGS. 8 and 10. The abscissa represents the ratio ofthe width B1, in the radial direction, of the aperture 12 to the beamdiameter A1. The ordinate represents the amount normalized with theamplitude of the tracking signal obtained when no aperture is arranged(B1/A1=1). As B1/A1 becomes smaller, the amplitude of the trackingsignal is lowered quadratically. For example, when B1/A1=0.7, theamplitude of the tracking signal is about 70% of that obtained when noaperture is arranged. This reveals that rays masked to reduce crosstalkcomponents from an adjacent track include components modulated by agroove crossing signal.

FIG. 18 shows the calculation results of a variation in offset of thetracking signal, which are obtained when the width B1, in the radialdirection, of the aperture 12 is varied and the objective lens isshifted in the radial direction. When a relatively nearby track is to beaccessed, the objective lens is often shifted in the radial direction.The abscissa represents the objective lens shift amount normalized withthe effective diameter (beam diameter A1) of the objective lens, and theordinate represents the offset of the tracking signal. As B1/A1 becomessmaller, the offset of the tracking signal is reduced. For example, whenB1/A1 is set to be about 0.7, almost no offset of the tracking signal isgenerated even when the objective lens is shifted in the radialdirection. Although this value slightly varies depending on the grooveshapes of discs, the same tendencies are observed. This is because evenwhen the objective lens is shifted in the radial direction, themovements of the light spots 20-1 and 20-2 on the photodetectors 7-1 and7-2 are limited by the aperture 12.

As described above, when some rays in the radial direction are masked bythe aperture 12 and it influences the servo signals since a singledetection system is used to detect the magneto-optical signal and theservo signals as in the optical system shown in FIG. 14, the aperture 12preferably has B1/A1=0.50 to 0.85. More preferably, the aperture 12 hasB1/A1=0.55 to 0.75. When such an aperture is inserted in the vicinity ofthe pupil of the light-receiving system, crosstalk components from anadjacent track can be effectively reduced, and a decrease in amplitudeof the tracking signal can be suppressed within an allowable range. Inaddition, even when the objective lens is shifted in the radialdirection, the offset of the tracking signal can be decreased.

FIGS. 19 and 20 show the entire magneto-optical system according to thesecond embodiment of the present invention. FIG. 19 is a front view of amagneto-optical disc recording/reproduction apparatus of the presentinvention, and FIG. 20 is a circuit diagram for explaining a method ofdetecting a magneto-optical signal. The same reference numerals in FIG.19 denote parts having the same functions as in FIG. 15, and a detaileddescription thereof will be omitted.

Referring to FIG. 19, a light beam emitted from a semiconductor laser 1is collimated by a collimator lens 2. The light beam incident on apolarization beam splitter 3 emerges from a stationary portion opticalsystem 29 toward an optical head movable portion 16, and forms a finelight spot 9 on a magneto-optical disc 5 by an objective lens 4.

The light beam, which is reflected by the recording medium surface andis incident again on the objective lens 4, is reflected by thepolarization beam splitter 3 via a mirror 15, and is guided toward asignal detection system. A polarization beam splitter 30 splits theoptical path into two paths for a magneto-optical signal detectionsystem and a servo signal detection system. The light beam reflected bythe polarization beam splitter 30 is guided toward a photodetector 31via a condenser lens 6 and a cylindrical lens 19. The photodetector 31comprises a quadrant sensor, and obtains a focusing error signal and atracking error signal by the arrangement shown in FIG. 16 (not shown).Since an aperture 12 is not arranged between the objective lens 4 andthe condenser lens 6, a normal circular light spot is formed on thephotodetector 31.

The light beam transmitted through the polarization beam splitter 30passes through the aperture 12 of the present invention via a halfwaveplate 17, and some marginal rays in the radial direction are masked. Theaperture 12 is arranged in the vicinity of the pupil of thelight-receiving system, as described above. The light beam transmittedthrough or reflected by a polarization beam splitter 18 is guided ontophotodetectors 7-1 and 7-2.

Reproduction of a magneto-optical signal will be explained below withreference to FIG. 20. FIG. 20 illustrates a state wherein the light beamreflected by the polarization beam splitter 18 forms light spots 20-1and 20-2 on the photodetectors 7-1 and 7-2 via the condenser lens 6 andthe cylindrical lens 19. Since marginal rays in the radial direction aremasked by the aperture 12, each light spot has a shape, as shown in FIG.20. A magneto-optical signal 28 is detected by differentially amplifyingsum outputs of the photodetectors 7-1 and 7-2 by a differentialamplifier 25.

When the aperture 12 does not influence the servo signals sinceindependent detection systems are arranged to detect the magneto-opticalsignal and the servo signals as in the optical system shown in FIG. 19,the aperture 12 preferably has B1/A1=0.40 to 0.85. More preferably, theaperture 12 has B1/A1=0.45 to 0.75. In this case, a drop of the C/N(carrier to noise) ratio due to a decrease in carrier level uponinsertion of the aperture need only be considered. As is apparent from acomparison between FIGS. 8 and 9, even when B1/A1 is set to be a smallvalue, the decrease in carrier level is smaller than the decrease inamplitude of the tracking signal. The decrease in carrier level whenB1/A1=0.5 is about 5 dB. While the decrease in C/N ratio can be as smallas about 1.5 dB, it depends on a magneto-optical disc to be reproducedand the characteristics of an optical head used. When such an apertureis inserted in the vicinity of the pupil of the light-receiving system,crosstalk components from an adjacent track can be effectively reduced.

FIGS. 21 and 22 show the third embodiment of the present invention. FIG.21 is a front view of an aperture of the present invention, and FIG. 22shows the computer simulation results of the reduction effect ofcrosstalk components from an adjacent track in an optical system whichuses the aperture according to the third embodiment of the presentinvention.

FIG. 21 is a front view of an elliptical aperture of the presentinvention. In the above description, a rectangular aperture shown inFIG. 7 is used. However, the effect of the present invention can besimilarly expected with other aperture shapes. FIG. 21 shows anelliptical aperture, which has a dimension B in the radial direction(B1<A1), and a dimension C1 in the track direction perpendicular to theradial direction. C1 may be equal to or larger than A1.

FIG. 22 shows the calculation results of variations in crosstalkcomponents from the adjacent track and the reproduction signal(carrier), which are obtained by varying the width B1, in the radialdirection, of the elliptical aperture. The dimension C1 in the trackdirection is set to be equal to A1. The calculation conditions are thesame as those in FIGS. 8 and 10. The abscissa represents the ratio ofthe width B1, in the radial direction, of an aperture 12 to the beamdiameter A1. The ordinate represents the amount normalized with thecarrier or the crosstalk component from an adjacent track obtained whenno aperture is arranged (B1/A1=1). When B1/A1 is varied, the reductioneffect appears when B1/A1=0.9 or less. For example, when B1/A1=0.7, thedecrease in carrier amount is about 2 dB, while the decrease incrosstalk amount from an adjacent track is about 7 dB. Thus, the effectis remarkable. FIG. 22 also shows the crosstalk amount to the carrier,which is normalized with a value obtained without an aperture, and areduction effect of about 5 dB is confirmed.

Assume a case wherein such an elliptical aperture is applied to anoptical system which includes a single detection system to detect amagneto-optical system and servo signals as in FIG. 15 in place of therectangular aperture 12. Since an elliptical aperture masks more rays inthe radial direction than those masked by a rectangular aperture, aninfluence on the servo signals becomes slightly large. The ellipticalaperture preferably has B1/A1=0.55 to 0.90. More preferably, theelliptical aperture has B1/A1=0.60 to 0.80. When such an aperture isinserted in the vicinity of the pupil of the light-receiving system,crosstalk components from an adjacent track can be effectively reduced,and a decrease in amplitude of the tracking signal can be suppressedwithin an allowable range. In addition, even when the objective lens isshifted in the radial direction, almost no offset of the tracking signalis generated.

On the other hand, when an elliptical aperture is used in place of therectangular aperture 12 in the optical system shown in FIG. 19, sinceindependent detection systems are arranged to detect the magneto-opticalsignal and the servo signals, the elliptical aperture does not influencethe servo signals. Since an elliptical aperture masks more rays in thetrack direction than those masked by a rectangular aperture, aninfluence on the reproduction signal becomes slightly large. Therefore,the aperture 12 preferably has B1/A1=0.45 to 0.90. More preferably, theaperture has B1/A1=0.50 to 0.80. When such an elliptical aperture isinserted in the vicinity of the pupil of the light-receiving system,crosstalk components from an adjacent track can be effectively reduced.

FIGS. 23 and 24 show the entire magneto-optical head optical systemaccording to the fourth embodiment of the present invention. FIG. 23 isa front view of a magneto-optical disc recording/reproduction apparatusof the present invention, and FIG. 24 is a circuit diagram forexplaining a method of detecting a magneto-optical signal. The samereference numerals in FIG. 23 denote parts having the same functions asin FIG. 15, and a detailed description thereof will be omitted.

Referring to FIG. 23, a light beam emitted from a semiconductor laser 1is collimated by a collimator lens 2. The light beam incident on apolarization beam splitter 3 emerges from a stationary portion opticalsystem 29 toward an optical head movable portion 16, and forms a finelight spot 9 on a magneto-optical disc 5 by an objective lens 4.

The light beam, which is reflected by the recording medium surface andis incident again on the objective lens 4, is reflected by thepolarization beam splitter 3 via a mirror 15, and is guided toward asignal detection system. A polarization beam splitter 30 splits theoptical path into two paths for a magneto-optical signal detectionsystem and a servo signal detection system. The servo signal detectionsystem has the same arrangement as that shown in FIG. 19. The light beamtransmitted through the polarization beam splitter 30 is transmittedthrough or reflected by a polarization beam splitter 18 via a halfwaveplate 17, and is guided toward photodetectors 7-1 and 7-2. Each of thephotodetectors 7-1 and 7-2 has a dimension in the radial directionsmaller than the beam diameter.

This will be described in detail below with reference to FIG. 24. Sincethe dimension, in the radial direction, of the light-receiving portionof each of the photodetectors 7-1 and 7-2 is smaller than the beamdiameter, marginal rays in the radial direction can be prevented frombeing detected without using the above-mentioned aperture. Thisarrangement can provide a reduction effect of crosstalk components froman adjacent track, which is equivalent to that obtained when therectangular aperture 12 is used.

A magneto-optical signal 28 is reproduced by differentially amplifyingthe sum outputs from the photodetectors 7-1 and 7-2 by a differentialamplifier 25.

Since the optical system shown in FIG. 23 includes independent detectionsystems to detect the magneto-optical signal and the servo signals, eachphotodetector preferably has B1/A1=0.40 to 0.85 where B1 is thedimension, in the radial direction, of the light-receiving portion ofeach photodetector in place of the aperture. More preferably, eachphotodetector has B1/A1=0.45 to 0.75. When photodetectors with suchdimensions are used, crosstalk components from an adjacent track can beeffectively reduced.

The arrangement according to the fifth embodiment of the presentinvention will be described below with reference to FIGS. 25 to 27. FIG.25 is a sectional view, in the radial direction, of a magneto-opticaldisc 5 in an optical system of the present invention, FIG. 26 is asectional view, in the track direction, of the disc 5, and FIG. 27 is aview showing the shape of an aperture 12'. FIGS. 25 and 26 particularlyillustrate only the light-receiving system for explaining the principleof the present invention.

Referring to FIG. 25, the magneto-optical disc 5 is tilted in the radialdirection indicated by an arrow 13, and a state wherein a side lobe dueto a coma caused by the tilt being generated in a light spot 9 isillustrated beside the light-receiving system. A light spot isillustrated in a state observed in the direction of its rays, and itsa-a' section corresponds to the radial direction.

Returned light from the magneto-optical disc 5 is collimated via anobjective lens 4, and is guided to a photodetector 7 via a condenserlens 6. In FIG. 5, the aperture 12 is arranged between the objectivelens 4 and the condenser lens 6, i.e., in the vicinity of the pupil ofthe light-receiving system. However, an aperture 12' of this embodimentis arranged between the condenser lens 6 and the photodetector 7 andmasks marginal rays in the radial direction, so that these rays do notreach the photodetector 7. Since the aperture 12' is separated by adistance d from a focal point position 10 of the condenser lens 6, itsposition corresponds to the far field region sufficiently separated fromthe vicinity of the focal point. The photodetector 7 is arranged behindthe focal point position 10 for the sake of simplicity, but may bearranged in front of or at the focal point position.

FIG. 25 illustrates rays which pass various height positions of thepupil diameter. As can be seen from FIG. 25, of these rays, hatched raysmasked by the aperture 12' mainly form a side lobe of a light spot onthe disc. Therefore, most of crosstalk components from an adjacent trackare included in the marginal rays, and if these rays are masked, thecrosstalk components can be reduced. The side lobe due to the coma andmarginal rays in the far field region of the light-receiving systemnormally have a good correspondence therebetween although they do nothave a strict one-to-one correspondence therebetween unlike that betweenthe light spot on the disc and the light spot of the light-receivingsystem shown in FIG. 4.

Referring to FIG. 26, the magneto-optical disc 5 is not tilted in thetrack direction perpendicular to the radial direction indicated by anarrow 14. Similarly, FIG. 26 illustrates, beside the light-receivingsystem, a state wherein a side lobe due to a coma is generated in thelight spot 9. The b-b' section of the light spot corresponds to thetrack direction. Returned light from the magneto-optical disc 5 iscollimated via the objective lens 4, and is guided toward thephotodetector 7 via the condenser lens 6. The aperture 12' has adimension in the track direction larger than a beam diameter A1 so asnot to guide modulated components of a reproduction signal from the discto the photodetector as much as possible since these components arespatially distributed in the track direction in the far field region ofthe light-receiving system.

FIG. 27 is a front view of the aperture 12'. The dimension in the radialdirection is B1' (B1'<A1'), and the dimension in the track direction isC1' (C1'>A1'). When the objective lens has a focal length fo=3 mm andNA=0.55, if the aperture 12' is arranged to be sufficiently separatedfrom the focal point position 10 of the condenser lens, for example, ifd=3 mm, a beam diameter A1'=0.33 mm. As will be described later, sinceB1' is selected to fall within a range from about 0.13 to 0.28 mm, easyadjustment of the aperture is greatly improved as compared to the priorart in which a pinhole having a diameter of 15 μm is inserted in thefocal plane of the light-receiving system, and the aperture is noteasily influenced by a change in temperature or aging. Since theaperture 12' is inserted in the light-receiving system, a considerabledecrease in light utilization efficiency due to the insertion of anannular aperture can be prevented unlike in the prior art, and thisarrangement is suitable for, e.g., a magneto-optical discrecording/reproduction apparatus which requires high power uponrecording/erasing of information.

The distance by which the aperture is separated from the focal pointposition 10 of the condenser lens of the light-receiving system and withwhich the far field region sufficiently separated from the vicinity ofthe focal point is determined will be examined below. In other words,the value of d, with which the effect of the present invention isexpected, unlike in the prior art shown in FIG. 3, will be examinedbelow with reference to FIG. 28.

FIG. 28 shows the shapes of the light spot when the position of theobservation is gradually separated (defocused) from the vicinity of thefocal point. This illustration is quoted from Hiroshi Kubota, "WaveOptics," p. 317. FIG. 28 illustrates the shapes of the light spot, whichcorrespond to wave aberrations from 0 to 1.5λ at λ/4 intervals.

Referring to FIG. 28, the abscissa represents a value in units of 2πNA/λwhere λ is the wavelength of a light beam from the semiconductor laser1, and NA is the numerical aperture of the objective lens 4. Morespecifically, the abscissa in FIG. 28 represents a value obtained bymultiplying the abscissa in FIG. 2 with 2π. The light spot shapeexpressed by the curve a in FIG. 2 is the same as that obtained whendefocus=0 in FIG. 28. The ordinate represents a value normalized withthe central intensity of the light spot obtained when defocus=0. For thesake of simplicity, a case will be examined below wherein a light beamhaving an almost uniform intensity distribution is incident on theobjective lens.

From the following discussion, a region corresponding to a defocusamount=λ or more can be considered to be the far field regionsufficiently separated from the vicinity of the focal point.

First, upon observation of the light spot shape when a defocus amount=λin FIG. 28, the central portion has an intensity of almost 0, and thislight spot shape is considerably different from that in an in-focusstate. In this state, no effect can be expected even when the pinholelike in the prior art shown in FIG. 3 is arranged. Therefore, a regioncorresponding to a defocus amount=λ or more can be considered to be thefar field region sufficiently separated from the vicinity of the focalpoint.

Second, if a region where a push-pull signal upon crossing of groovescan be stably detected is defined to be the far field region, such aregion has a defocus amount of λ or more from experimental results. Whena push-pull signal is detected in a region having a defocus amount of λor less, a variation in light intensity distribution becomes criticalwith respect to even a slight variation in defocus amount, and stabletracking cannot be assured.

For example, in the optical system shown in FIG. 25, if the focallengths of the objective lens and the condenser lens are set to be fo=3mm and fc=30 mm, the NA of the objective lens is set to be 0.55, and thewavelength of the semiconductor laser is set to be λ=780 nm, a waveaberration W20 caused by a defocus amount in the light-receiving systemis given by:

    W20=d/{8·(fc/A).sup.2 ·}                 (1)

From equation (1), in the light-receiving system, a region with d=0.5 mmor more corresponds to the far field region. Since a region whichsatisfies W20≧1 (λ) corresponds to the far field region, the aperturecan be arranged at a position separated from the focal plane by thedistance d or more given by the following equation:

    d=8·(FNo).sup.2 ·λ

where FNo is the effective f-number of the light-receiving system, and λis the wavelength of a light beam from a light source.

FIGS. 29 and 30 show the entire magneto-optical head optical systemaccording to the fifth embodiment of the present invention. FIG. 29 is afront view of a magneto-optical disc recording/reproduction apparatus ofthe present invention, i.e., of an optical head which uses a singledetection system to detect a magneto-optical signal and servo signals.The same reference numerals in FIG. 29 denote parts having the samefunctions as in FIG. 15, and a detailed description thereof will beomitted.

Referring to FIG. 29, a light beam emitted from a semiconductor laser 1is collimated by a collimator lens 2. The light beam incident on apolarization beam splitter 3 emerges from a stationary portion opticalsystem 29 toward an optical head movable portion 16, and forms the finelight spot 9 on the magneto-optical disc 5 by the objective lens 4.

The light beam, which is reflected by the recording medium surface andis incident again on the objective lens 4, is reflected by thepolarization beam splitter 3 via a mirror 15, and is guided toward asignal detection system. The light beam transmitted through a halfwaveplate 17 is transmitted through or reflected by a polarization beamsplitter 18. The split light beams respectively pass through apertures12'-1 and 12'-2 and some marginal rays in the radial direction aremasked. The light beams are then guided onto photodetectors 7-1 and 7-2.The apertures 12'-1 and 12'-2 are arranged to be separated by a distanced from focal points 10-1 and 10-2 of the light-receiving system. Thephotodetectors 7-1 and 7-2 are arranged in the vicinity of the apertures12'-1 and 12'-2.

The light beam reflected by the polarization beam splitter 3 forms lightspots 20-1 and 20-2 on the photodetectors 7-1 and 7-2 via the condenserlens 6 and a cylindrical lens 19 as in FIG. 16. In order to detect afocusing error signal by an astigmatism method, the photodetector 7-1 isarranged in the vicinity of the circle of least confusion, and a lightspot on the photodetector normally has a circular shape. In this case,however, since marginal rays in the radial direction are masked by theapertures 12'-1 and 12'-2, the light spot has the light spot shape asshown in FIG. 16. Reproduction of the magneto-optical signal and thedetection system for the servo signals are the same as those in FIG. 16.

In this embodiment, the optical system is constituted as follows. Thecondenser lens comprises a plano-convex lens and the cylindrical lenscomprises a plano-concave lens.

    ______________________________________                                                                          Glass                                                   R1   R2         D     Material                                    ______________________________________                                        Condenser lens 6                                                                            14.73  ∞    2.0 BSL7                                      Cylindrical Lens 19                                                                         ∞                                                                              129.65     1.5 BSL7                                      ______________________________________                                    

In this table, R1 is the radius of curvature of the first surface, R2 isthe radius of curvature of the second surface, and D is the lensthickness (mm). The air gap between the condenser lens 6 and thecylindrical lens 19 is 5.9 mm.

If the wavelength of the semiconductor laser 1 is λ=780 nm, the focallength, in the meridional direction, of the cylindrical lens is fm=28.87mm, the focal length in the sagittal direction is fs=31.43 mm, and anastigmatic difference As=1.83 mm is generated. If the objective lens hasa focal length fo=3 mm and NA=0.55, the beam diameter A=3.3 mm. A waveaberration W22 due to an astigmatism is given by:

    W22=As/{8·((fm+fs)/2A).sup.2 ·λ}  (2)

From the above equation, W22=3.5λ. At the position of the circle ofleast confusion where the aperture and the photodetector are arranged,the following defocuses are obtained from focal lines in the meridionaland sagittal directions of the cylindrical lens 19:

    W20 (meridional direction)=(As-d)/{8·fm/A).sup.2 ·λ}                                       (3)

    W20 (sagittal direction)=-d/{8·(fs/A).sup.2 ·λ}(4)

    d=As·fs/(fm+fs)                                   (5)

From the above equations, a defocus W20 (meridional direction)=1.8λ anda defocus W20 (sagittal direction)=-1.7λ are obtained. From thediscussion with reference to FIG. 28, this position can be considered tobe the far field region sufficiently separated from the vicinity of thefocal point. At this position, when marginal rays in the radialdirection are masked by the apertures 12'-1 and 12'-2, crosstalkcomponents from an adjacent track can be effectively reduced, and astable tracking signal can be obtained.

When an astigmatism method is used in detection of a focusing errorsignal, the astigmatism W22 is set to be at least 2λ or more, and ispreferably set to be 3λ to 10λ in consideration of the focus capturerange. The defocus W20 from the focal lines at the position of thecircle of least confusion where the aperture and the photodetector arearranged is W20=±W22/2, and is set to be at least λ or more and is morepreferably set to be about 1.5λ to 5λ. When the astigmatism and defocusare selected in this manner, crosstalk components from an adjacent trackcan be effectively reduced by masking marginal rays in the radialdirection by the aperture, and a stable tracking signal can be obtained.

Since the optical system shown in FIG. 29 includes a single detectionsystem to detect a magneto-optical signal and servo signals, somemarginal rays in the radial direction are masked by the apertures 12'-1and 12'-2 and the apertures influence servo signals. In this embodiment,since the beam diameter A1' at the arrangement position of the apertureis about 100 μm, the value of the width B1', in the radial direction, ofthe aperture is decreased accordingly. However, as in the firstembodiment, the value of the width B1', in the radial direction, of theaperture preferably satisfies B1'/A1'=0.50 to 0.85. More preferably, B1'satisfies B1'/A1'=0.55 to 0.75. When such apertures are inserted in thefar field region sufficiently separated from the vicinity of the focalpoint of the light-receiving system, crosstalk components from anadjacent track can be effectively reduced, and a decrease in amplitudeof the tracking signal can be suppressed within an allowable range. Inaddition, even when the objective lens is shifted in the radialdirection, the offset of the tracking signal can be decreased.

FIG. 30 is a front view of a magneto-optical recording/reproductionapparatus of the present invention, i.e., an optical head which includesindependent detection systems to detect a magneto-optical signal andservo signals. The same reference numerals in FIG. 30 denote partshaving the same functions as in FIG. 15 and a detailed descriptionthereof will be omitted.

Referring to FIG. 30, a light beam emitted from the semiconductor laser1 is collimated by the collimator lens 2. The light beam incident on thepolarization beam splitter 3 emerges from the stationary portion opticalsystem 29 toward the optical head movable portion 16, and forms the finelight spot 9 on the magneto-optical disc 5 via the objective lens 4.

The light beam, which is reflected by the recording medium surface andis incident again on the objective lens 4, is reflected by thepolarization beam splitter 3 via the mirror 15, and is guided toward asignal detection system. A polarization beam splitter 30 splits theoptical path into two paths for a magneto-optical signal detectionsystem and a servo signal detection system. The light beam reflected bythe polarization beam splitter 30 is guided toward a photodetector 31via a condenser lens 6 and a cylindrical lens 19. The photodetector 31comprises a quadrant sensor, and obtains a focusing error signal and atracking error signal by the arrangement shown in FIG. 16 (not shown).Since no aperture is arranged between the objective lens 4 and thecondenser lens 6, a normal circular light spot is formed on thephotodetector 31.

The light beam transmitted through the polarization beam splitter 30 isincident on the halfwave plate 17 and the condenser lens 6 in turn.Thereafter, the light beam is transmitted through or reflected by thepolarization beam splitter 18. The split light beams pass through theapertures 12'-1 and 12'-2 of the present invention, and some marginalrays in the radial direction are masked. The light beams are then guidedonto the photodetectors 7-1 and 7-2. The apertures 12'-1 and 12'-2 arearranged to be separated by a distance d from focal points 10-1 and 10-2of the light-receiving system. The photodetectors 7-1 and 7-2 arearranged at appropriate positions. In FIG. 30, although thephotodetectors 7-1 and 7-2 are arranged in front of the correspondingfocal point position 10, they may be arranged behind or at the focalpoint positions. Reproduction of the magneto-optical signal is the sameas that in FIG. 20.

In this embodiment, the optical system is arranged as follows. The focallengths of the objective lens and the condenser lens are set to be fo=3mm and fc=30 mm, the NA of the objective lens is set to be 0.55, thewavelength of the semiconductor laser is set to be λ=780 nm, the beamdiameter A=3.3 mm, and d=-1.0 mm. From equation (1), the apertures 12'-1and 12'-2 are arranged at positions with the defocus W20=-2λ. From thediscussion in FIG. 28, this position can be considered to be the farfield region sufficiently separated from the vicinity of the focalpoint. At this position, by masking marginal rays in the radialdirection by the apertures 12'-1 and 12'-2, crosstalk components from anadjacent track can be effectively reduced. The defocus W20 is set to beat least λ or more, and is preferably set to be 1.5λ or more.

Since the optical system shown in FIG. 30 includes independent detectionsystems to detect the magneto-optical signal and the servo signals, theapertures 12'-1 and 12'-2 do not influence the servo signals. In thisembodiment, since the beam diameter A1' at the arrangement position ofthe aperture is about 110 μm, the value of the width B1', in the radialdirection, of the aperture is decreased accordingly. However, as in thesecond embodiment, the value of the width B1', in the radial direction,of the aperture preferably satisfies B1'/A1'=0.40 to 0.85. Morepreferably, B1' satisfies B1'/A1'=0.45 to 0.75. When such apertures areinserted in the far field region sufficiently separated from thevicinity of the focal point of the light-receiving system, crosstalkcomponents from an adjacent track can be effectively reduced.

FIGS. 31 to 33 show the entire magneto-optical head optical systemaccording to the sixth embodiment of the present invention. FIG. 31 is afront view of a magneto-optical disc recording/reproduction apparatus ofthe present invention, FIG. 32 is a side view of the magneto-opticaldisc recording/reproduction apparatus of the present invention, and FIG.33 is a circuit diagram for explaining a method of detecting amagneto-optical signal and servo signals. The same reference numerals inFIG. 31 denote parts having the same functions as in FIG. 15, and adetailed description thereof will be omitted.

In this embodiment, the present invention is applied to an overwritecapable magneto-optical disc recording/reproduction apparatus which canperform recording/erasing/reproduction during one revolution of a disc.A magneto-optical disc 5 is a recording medium which allows an overwriteoperation by magnetic field modulation or light intensity modulationrecording. The magneto-optical head optical system shown in FIG. 31forms two light spots on a single track to perform verificationimmediately after recording. The difference from the above embodimentsis that a semiconductor laser array 1' is used to obtain a plurality ofbeams in place of the semiconductor laser 1. The light-emitting pointsof the semiconductor laser array 1' are aligned in the plane of thedrawing of FIG. 32.

A plurality of light beams emitted from the semiconductor laser array 1'are collimated by a collimator lens 2. The light beams incident on apolarization beam splitter 3 emerge from a stationary portion opticalsystem 29 toward an optical head movable portion 16. The light beamsreflected by a mirror 15 form two light spots 34 and 35 on a track 33via an objective lens 4. An arrow 36 indicates the rotational directionof the magneto-optical disc 5, and information overwritten by therecording/erasing light spot 34 is immediately reproduced by theverification light spot 35. The optical head movable portion 16 ismovable in the directions of the arrows shown in FIG. 31.

The light beams, which are reflected by the recording medium surface andare incident on the objective lens 4 again, are reflected by thepolarization beam splitter 3 via the mirror 15, and are guided toward asignal detection system. The light beams incident on a polarization beamsplitter 32 via a halfwave plate 17 and a condenser lens 6 aretransmitted therethrough or reflected thereby, and are guided onto aphotodetector 7. The photodetector 7 is arranged in front of the focalpoint of the condenser lens 6 by a distance d with respect to lightbeams transmitted through the polarization beam splitter 32, and isarranged behind the focal point by a distance d with respect to lightbeams reflected by the polarization beam splitter 32. The photodetector7 has a plurality of light-receiving portions on a single substrate.

Detection systems of a magneto-optical signal and servo signals will beexplained below with reference to FIG. 33. FIG. 33 shows a state whereinthe light beams transmitted through or reflected by the polarizationbeam splitter 32 form light spots 20-1, 20-2, 37-1, and 37-2 onphotodetectors 7-1, 7-2, 7-3, and 7-4. The light spots 37-1 and 37-2correspond to light beams from the recording/erasing light spot 34 onthe recording medium, and the light spots 20-1 and 20-2 correspond tolight beams from the verification light spot 35. In this embodiment,servo signals are detected based on the light beams from therecording/erasing light spot 34, and a magneto-optical signal isreproduced based on light beams from the verification light spot 35. Forthis purpose, the photodetectors 7-3 and 7-4 respectively compriselight-receiving portions 7-a to 7-d and 7-e to 7-h.

Detection of a focusing error signal will be described below. Since thephotodetectors 7-3 and 7-4 are arranged before and after the focal pointof the condenser lens 6 to be separated therefrom by the distance d, ifa defocus is generated in a light spot 9 on the magneto-optical disc,the diameter of one light spot (e.g., 37-1) increases, and the diameterof the other light spot (e.g., 37-2) decreases. Therefore, bycalculating a difference between the sum outputs from the innerlight-receiving portions of the photodetectors, a focusing error signalcan be detected. More specifically, a sum output of the light-receivingportions 7-a and 7-b is generated by an adder 23-2, a sum output of thelight-receiving portions 7-e and 7-f is generated by an adder 23-1, andthese outputs are differentially amplified by a differential amplifier21, thereby obtaining a focusing error signal 26. Such a method ofdetecting the focusing error will be referred to as a differential beamsize method hereinafter. Note that the sum output of the light-receivingportions 7-a and 7-b may be normalized with the sum of the outputs ofthe photodetector 7-3 as a whole, the sum output of the light-receivingportions 7-e and 7-f may be normalized with the sum of the outputs ofthe photodetector 7-4 as a whole, and thereafter, these outputs may bedifferentially amplified to obtain an output which is not easilyinfluenced by the presence/absence of a magneto-optical signal or thebirefringence of a substrate (not shown).

Detection of a tracking error signal will be described below. Push-pullsignals are obtained from difference outputs of the innerlight-receiving portions of the photodetectors 7-3 and 7-4, and adifference between these signals is calculated. More specifically, adifference output of the light-receiving portions 7-a and 7-b isgenerated by a differential amplifier 22-2, a difference output of thelight-receiving portions 7-e and 7-f is generated by a differentialamplifier 22-1, and these outputs are differentially amplified by adifferential amplifier 24 to obtain a tracking error signal 27. Thus, atracking signal which is not easily influenced by an optical axis shiftdue to a change in temperature or aging can be obtained.

A magneto-optical signal 28 is detected by differentially amplifying theoutputs from the photodetectors 7-1 and 7-2 by a differential amplifier25. Each of the photodetectors 7-1 and 7-2 has a dimension in the radialdirection smaller than the beam diameter as in the fourth embodiment.Since the dimension, in the radial direction, of the light-receivingportion is smaller than the beam diameter, marginal rays can beprevented from being detected without using an aperture. Thisarrangement can provide a reduction effect of crosstalk components froman adjacent track, which is equivalent to that obtained when therectangular aperture 12 is used.

In this embodiment, the optical system is arranged as follows. The focallengths of the objective lens and the condenser lens are set to be fo=3mm and fc=30 mm, the NA of the objective lens is set to be 0.55, thewavelength of the semiconductor laser is set to be λ=780 nm, the beamdiameter A=3.3 mm, and the distance d between the focal point of thecondenser lens and each photodetector is set to be d=±1.0 mm. Fromequation (1), the photodetectors 7-1 to 7-4 are arranged at positionswith the defocus W20=±2λ. From the discussion in FIG. 28, thesepositions can be considered to be the far field region sufficientlyseparated from the vicinity of the focal point. At these positions,since marginal rays in the radial direction are not received, crosstalkcomponents from an adjacent track can be effectively reduced. When thedifferential beam size method is used in detection of the focusing errorsignal, the defocus W20 is set to be at least λ or more, and preferably,about 1.5λ to 5λ in consideration of a focus capture range. In thismanner, when the photodetector positions are selected to have thedefocus W20, since marginal rays in the radial direction are notreceived, crosstalk components from an adjacent track can be effectivelyreduced, and a stable tracking signal can be obtained.

Since the beam diameter A1' at the arrangement position of the apertureis about 110 μm, the value of the width B1', in the radial direction, ofthe aperture is decreased accordingly. As in the second embodiment, thevalue of the width B1', in the radial direction, of the light-receivingportion of each photodetector preferably satisfies B1'/A1'=0.40 to 0.85in place of the aperture. More preferably, B1' satisfies B1'/A1'=0.45 to0.75. When photodetectors with such dimensions are used, crosstalkcomponents from an adjacent track can be effectively reduced.

In the optical system of this embodiment, detection systems for amagneto-optical signal and servo signals can be independentlyconstituted using two beams although most optical system components arecommonly used. For this reason, crosstalk reduction from an adjacenttrack does not influence the servo signals. In addition, since detectionoptical systems for a magneto-optical signal and servo signals need notbe independently arranged, an optical head can be rendered compact, andits cost can be reduced.

As described above, when an aperture for masking marginal rays, in theradial direction, of returned light from a disc is arranged in a regionsufficiently separated from the focal plane of the optical headlight-receiving system, e.g., in the vicinity of the pupil of thelight-receiving system, the influence of crosstalk from an adjacenttrack can be effectively reduced. Also, the same effect can be expectedby masking marginal rays, in the radial direction, of returned light bymodifying the shape of a photodetector. When the present invention isused, easy adjustment of the aperture or the photodetector is greatlyimproved as compared to the prior art in which a pinhole is inserted inthe focal plane of the light-receiving system, and the arrangement ofthe present invention is not easily influenced by a change intemperature or aging. Since the aperture is inserted in thelight-receiving system, a considerable decrease in light utilizationefficiency due to the insertion of an annular aperture can be preventedunlike in the prior art, and the arrangement of the present invention issuitable for, e.g., a magneto-optical disc recording/reproductionapparatus which requires high power upon recording/erasing ofinformation.

Each of the above embodiments has as its object to effectively reducethe influence of crosstalk from an adjacent track, but each of theembodiments to be described below has as its object to effectivelyreduce information reproduced from an adjacent mark on a single track.

More specifically, in an optical recording/reproduction apparatus whichirradiates a light beam from a light source onto a predetermined trackon an optical recording medium to form a fine light spot, andtime-serially scans a plurality of marks located on the track to performrecording/reproduction of information or reproduction of information,mask means for masking marginal rays, in the track direction, of areturned light beam from the optical recording medium is arranged in thefar field region sufficiently separated from the focal plane of adetection optical system for detecting the returned light beam in theoptical path of the detection optical system, thereby reducinginformation reproduced from a mark adjacent to a predetermined mark on asingle track upon reproduction of the predetermined mark located on thetrack.

The arrangement of the seventh embodiment according to the presentinvention will be described below with reference to FIGS. 34 to 36. FIG.34 is a sectional view, in the track direction, of a magneto-opticaldisc 5 in an optical system of the present invention, FIG. 35 is asectional view, in the radial direction, of the disc 5, and FIG. 36 is aview showing the shape of an aperture 38. FIGS. 34 and 35 particularlyillustrate only the light-receiving system for explaining the principleof the present invention.

Referring to FIG. 34, the magneto-optical disc 5 is tilted in the trackdirection indicated by an arrow 14, and a state wherein a side lobe dueto a coma is generated in a light spot 9 by the tilt of the disc 5 isillustrated beside the light-receiving system. A light spot isillustrated in a state observed in the direction of its rays, and itsb-b' section corresponds to the track direction. When information on agiven track is reproduced in this state, the side lobe caused by thecoma undesirably reproduces information of an adjacent mark, and thereproduced information generates a distortion or shift in a reproductionsignal to be originally reproduced. This is called an intersymbolinteraction, and causes an increase in jitter. In particular, in a discconsisting of a plastic material such as polycarbonate, the problem ofan increase in jitter caused by a tilt of the disc is unavoidable, andseriously disturbs an increase in density.

Returned light from the magneto-optical disc 5 is collimated via anobjective lens 4, and is guided to a photodetector 7 by a condenser lens6. An aperture 38 is arranged between the objective lens 4 and thecondenser lens 6, i.e., in the vicinity of the pupil of thelight-receiving system, and masks marginal rays in the track direction,so that these rays do not reach the photodetector. FIG. 34 illustratesrays which pass various height positions of the pupil diameter. As canbe seen from FIG. 34, of these rays, hatched rays masked by the aperture38 mainly form a side lobe of a light spot on the disc. Therefore, mostof the information components of an adjacent mark are included in thesemarginal rays, and the intersymbol interaction can be reduced by maskingthese rays. The side lobe due to the coma and marginal rays in thevicinity of the pupil of the light-receiving system normally have a goodcorrespondence therebetween although they do not have a strictone-to-one correspondence therebetween unlike that between the lightspot on the disc and the light spot of the light-receiving system shownin FIG. 4. The aperture 38 is arranged in the far field regionsufficiently separated from the focal plane of the light-receivingsystem, and masks marginal rays, in the track direction, of the returnedlight beam.

Referring to FIG. 35, the magneto-optical disc 5 is not tilted in thetrack direction indicated by the arrow 14. Similarly, FIG. 35illustrates, beside the light-receiving system, a state wherein a sidelobe due to a coma is generated in the light spot 9. The a-a' section ofthe light spot corresponds to the radial direction. Returned light fromthe magneto-optical disc 5 is collimated via the objective lens 4, andis guided toward the photodetector 7 via the condenser lens 6. Theaperture 38 has a dimension in the radial direction larger than the beamdiameter A2 so as not to mask marginal rays in the radial direction.This is to guide modulated components of a tracking signal from the discto the photodetector as much as possible since these components arespatially distributed in the radial direction in the pupil of thelight-receiving system.

FIG. 36 is a front view of the aperture 38. The dimension, in the trackdirection, of the aperture is B2 (B2<A2), and the dimension, in theradial direction, of the aperture is C2 (C2>A2). If the objective lens 4has a focal length fo=3 mm and NA=0.55, the beam diameter A2=3.3 mm. Aswill be described later, since the dimension B2 is selected to fallwithin a range of B2≈1.8 to 3.0 mm, easy adjustment of the aperture isgreatly improved as compared to the prior art in which a pinhole havinga diameter of 15 μm is inserted in the focal plane of thelight-receiving system, and the aperture is not easily influenced by achange in temperature or aging. Since the aperture 38 is inserted in thelight-receiving system, a considerable decrease in light utilizationefficiency due to the insertion of an annular aperture can be preventedunlike in the prior art, and this arrangement is suitable for, e.g., amagneto-optical disc recording/reproduction apparatus which requireshigh power upon recording/erasing of information.

From equation (1) described above, the aperture 38 can be arranged at aposition separated from the focal plane of the light-receiving system bya distance d given by the following equation:

    d=8·(FNo).sup.2 ·λ

where FNo is the effective f-number of the light-receiving system, and λis the wavelength of a light beam from a light source.

FIG. 37 shows the computer simulation results of a decrease in carrierin the optical system according to the seventh embodiment of the presentinvention. FIG. 37 shows the calculation results of a change inreproduction signal (carrier) by changing the width B2, in the trackdirection, of the aperture 38. The wavelength λ of the semiconductorlaser is set to be λ=780 nm, the tilt of the disc is 5 mrad. in thetrack direction, the objective lens has NA=0.55, the track pitch is 1.4μm, the track width of the recorded portion is 0.9 μm, and the carrierhas a mark length=0.75 μm. The abscissa represents the ratio of thewidth B2, in the track direction, of the aperture 38 to the beamdiameter A2. The ordinate represents the amount normalized with acarrier obtained when no aperture is arranged (B2/A2=1). When B2/A2 isdecreased, the carrier level is decreased gradually. This decrease incarrier is larger than that in a case wherein the width, in the radialdirection, of the aperture is changed, as shown in FIG. 8. This isbecause modulated components of a reproduction signal from a disc arespatially distributed in the track direction in the pupil of thelight-receiving system.

FIG. 38 shows the experimental results of the jitter reduction effectfor information (intersymbol interaction) reproduced from an adjacentmark on a single track in the optical system according to the seventhembodiment of the present invention. The experimental conditions are: amark edge recording method, a 1-7 modulation method for symbols, aminimum mark length=0.75 μm, and a linear velocity=15 m/s. FIG. 38 showscases obtained when the disc is not tilted and when the disc is tiltedat 5 mrad. in the track direction. The abscissa represents the ratio ofthe width B2, in the track direction, of the aperture 38 to the beamdiameter A2, and the ordinate represents the jitter amount. At no disctilt, the reduction effect is maximized when B2/A2 is set to be about0.75, and the effect is gradually reduced when B2/A2 is set to be equalto or lower than 0.75. This is for the following reason. That is,although the aperture 38 masks information from an adjacent mark, sincethe carrier is lowered as B2/A2 becomes smaller, the C/N ratiodeteriorates, and the level of jitter components due to noise increases.At 5 mrad. disc tilt, the reduction effect is maximized when B/A is setto be equal 0.72, which is rather lower than at no disc tilt, and theeffect is gradually reduced in the same manner.

FIGS. 39 and 40 show the entire magneto-optical head optical systemaccording to the seventh embodiment of the present invention. FIG. 39 isa front view of a magneto-optical disc recording/reproduction apparatusof the present invention, and FIG. 40 is a circuit diagram forexplaining a method of detecting a magneto-optical signal and servosignals. The same reference numerals in FIG. 39 denote parts having thesame functions in FIG. 15, and a detailed description thereof will beomitted.

The magneto-optical head optical system shown in FIG. 39 is a so-calledseparated optical system. A light beam emitted from the semiconductorlaser 1 is collimated by the collimator lens 2. The light beam incidenton the polarization beam splitter 3 emerges from a stationary portionoptical system 29 toward an optical head movable portion 16, and formsthe fine light spot 9 on the magneto-optical disc 5 by the objectivelens 4.

The light beam, which is reflected by the recording medium surface andis incident again on the objective lens 4, is reflected by thepolarization beam splitter 3 via a mirror 15, and is guided toward asignal detection system. The light beam transmitted through a halfwaveplate 17 passes through the aperture 38 of the present invention, andsome marginal rays in the track direction are masked. The aperture 38 isarranged between the objective lens 4 and the condenser lens 6, i.e., inthe vicinity of the pupil of the light-receiving system, as describedabove. The light beam transmitted through the condenser lens 6 and acylindrical lens 19 is transmitted through or reflected by apolarization beam splitter 18, and is guided onto photodetectors 7-1 and7-2.

A system for reproducing a magneto-optical signal and detecting servosignals will be explained below with reference to FIG. 40. FIG. 40illustrates a state wherein the light beam reflected by the polarizationbeam splitter 3 forms light spots 20-1 and 20-2 on the photodetectors7-1 and 7-2 via the condenser lens 6 and the cylindrical lens 19. Inorder to detect a focusing error signal by an astigmatism method, thephotodetector 7-1 is arranged in the vicinity of the circle of leastconfusion, and a light spot on the photodetector normally has a circularshape. In this case, however, since marginal rays in the track directionare masked by the aperture 38, the light spot has a shape as shown inFIG. 40.

Sums of diagonal photoelectric conversion outputs of the light spot 20-1on the photodetector 7-1 are calculated, and a difference between thesums is differentially amplified by a differential amplifier 21 togenerate a focusing error signal 26. A difference, in the radialdirection, of the outputs of the light spot 20-2 on the photodetector7-2 is differentially amplified by a differential amplifier 22 togenerate a tracking error signal (push-pull signal) 27. Amagneto-optical signal 28 is detected in such a manner that the sumoutputs of the photodetectors 7-1 and 7-2 are generated by sumamplifiers 23 and 24, and thereafter, a difference therebetween isdifferentially amplified by a differential amplifier 25. Note that thephotodetector 7-1 can simultaneously obtain a tracking error signal fromthe differential output in the radial direction since it adopts aquadrant sensor. Note that the optical system shown in FIG. 39 does notsuffer a decrease in amplitude of a tracking signal which poses aproblem in the optical system shown in FIG. 15 since rays in the radialdirection are not masked although a single detection system is used fordetecting a magneto-optical signal and servo signals.

As described above, in this embodiment, although some rays in the trackdirection are masked by the aperture 38, they do not influence servosignals. Therefore, regardless of whether a single detection signal isused or independent detection systems are used for detecting amagneto-optical signal and servo signals, the aperture 38 preferably hasB2/A2=0.55 to 0.90. More preferably, the aperture 38 has B2/A2=0.60 to0.85. When such an aperture is inserted in the vicinity of the pupil ofthe light-receiving system, information from an adjacent mark is masked,and jitter components due to an intersymbol interaction can be reduced.

As described above, when marginal rays in the track direction of thelight-receiving system are masked, information from an adjacent mark canbe masked, and jitter components due to an intersymbol interaction canbe reduced. As in the first to sixth embodiments in which marginal raysin the radial direction are masked to reduce crosstalk components fromthe adjacent track, means for masking marginal rays in the trackdirection may be arranged in the vicinity of the pupil of thelight-receiving system or in the far field region sufficiently separatedfrom the focal point of the light-receiving system. The shape of theaperture 38 may be a rectangle or an ellipse. Also, marginal rays in thetrack direction may be prevented from being received by modifying theshape of the photodetectors. Furthermore, they can be applied to anoptical system which includes a single detection system for amagneto-optical signal and servo signals, and an optical system whichincludes independent detection systems therefor. Note that reduction ofjitter components caused by a coma due to a disc tilt has beenparticularly exemplified. However, this embodiment is effective for acase wherein a spherical aberration or a defocus is generated.

When the present invention is adopted, easy adjustment of the apertureis greatly improved as compared to the prior art in which a pinhole isinserted in the focal plane of the light-receiving system, and thisarrangement is not easily influenced by a change in temperature oraging. Since the aperture is inserted in the light-receiving system, aconsiderable decrease in light utilization efficiency due to theinsertion of an annular aperture can be prevented unlike in the priorart, and this arrangement is suitable for, e.g., a magneto-optical discrecording/reproduction apparatus which requires high power uponrecording/erasing of information.

The arrangement according to the eighth embodiment of the presentinvention will be described below with reference to FIGS. 41 to 43. FIG.41 is a sectional view, in the track direction, of a magneto-opticaldisc 5 in an optical system of the present invention, FIG. 42 is asectional view, in the radial direction, of the disc 5, and FIG. 43 is aview showing the shape of an aperture 39. FIGS. 41 and 43 particularlyillustrate only the light-receiving system for explaining the principleof the present invention.

FIG. 41 illustrates, beside the light-receiving system, a state whereina side lobe due to a spherical aberration is generated in a light spot9. The spherical aberration is generated due to a manufacturing error ofthe objective lens and a substrate thickness error of the disc, and hasa side lobe which is symmetrical about the center of rotation. A lightspot is illustrated in a state observed in the direction of its rays,and its a-a' section corresponds to the radial direction. Wheninformation on a given track is reproduced in this state, the side lobecaused by the spherical aberration undesirably reproduces information onan adjacent track, and the reproduced information is found in areproduction signal as crosstalk components. When the NA of theobjective lens is to be increased, the manufacturing allowable errormust be reduced, and the problem of the crosstalk from an adjacent trackdue to a spherical aberration seriously disturbs an increase in density.

FIG. 42 similarly illustrates, beside the light-receiving system, astate wherein a side lobe due to a spherical aberration is generated ina state observed in the direction of its rays, and its b-b' sectioncorresponds to the track direction. When information on a given track isreproduced in this state, the side lobe caused by the sphericalaberration undesirably reproduces information on an adjacent mark, andthe reproduced information generates a distortion or shift in areproduction signal to be originally reproduced. Thus, the intersymbolinteraction undesirably increases, which, in turn, increases jitter.When the NA of the objective lens is to be increased, the manufacturingallowable error must be reduced, and the problem of the increase injitter due to a spherical aberration is unavoidable, thus seriouslydisturbing an increase in density.

Returned light from the magneto-optical disc 5 is collimated via anobjective lens 4, and is guided to a photodetector 7 by a condenser lens6. The aperture 39 is arranged between the objective lens 4 and thecondenser lens 6, i.e., in the vicinity of the pupil of thelight-receiving system, and masks marginal rays in the radial and trackdirections, so that these rays do not reach the photodetector 7.

FIGS. 41 and 42 illustrate rays which pass various height positions ofthe pupil diameter. As can be seen from FIGS. 41 and 42, of these rays,hatched rays masked by the aperture 39 mainly form a side lobe of alight spot on the disc. Therefore, most of the cross talk componentsfrom an adjacent track are included in marginal rays in the radialdirection, and can be reduced by masking these rays. On the other hand,most of the information components from an adjacent mark are included inthe marginal rays in the track direction, and the intersymbolinteraction can be reduced by masking these rays.

FIG. 43 is a front view of the aperture 39. The dimension, in the radialdirection, of the aperture is B1 (B1<A1), and the dimension, in thetrack direction, of the aperture is C1 (C1<A1). If the objective lens 4has a focal length fo=3 mm and NA=0.55, the beam diameter A1=3.3 mm.When B1 and C1 are selected to respectively fall within a range ofB1≈1.3 to 2.8 mm and a range of C1≠1.8 to 3.0 mm, crosstalk componentsfrom an adjacent track and jitter components due to intersymbolinteraction from an adjacent mark can be simultaneously reduced. In thisembodiment, easy adjustment of the aperture is greatly improved ascompared to the prior art in which a pinhole is inserted in the focalplane of the light-receiving system, and the aperture is not easilyinfluenced by a change in temperature or aging. Since the aperture 39 isinserted in the light-receiving system, a considerable decrease in lightutilization efficiency due to the insertion of an annular aperture canbe prevented unlike in the prior art, and this arrangement is suitablefor, e.g., a magneto-optical disc recording/reproduction apparatus whichrequires high power upon recording/erasing of information.

FIGS. 44 and 45 show the entire magneto-optical head optical systemaccording to the eighth embodiment of the present invention. FIG. 44 isa front view of a magneto-optical disc recording/reproduction apparatusof the present invention, and FIG. 45 is a circuit diagram forexplaining a method of detecting a magneto-optical signal. The samereference numerals in FIG. 44 denote parts having the same functions asin FIG. 15, and a detailed description thereof will be omitted.

Referring to FIG. 44, a light beam emitted from the semiconductor laser1 is collimated by a collimator lens 2. A light beam incident on apolarization beam splitter 3 emerges from a stationary portion opticalsystem 29 toward an optical head movable portion 16, and forms a finelight spot 9 on the magneto-optical disc 5 via the objective lens 4.

The light beam, which is reflected by the recording medium surface andis incident again on the objective lens 4, is reflected by thepolarization beam splitter 3 via a mirror 15, and is guided toward asignal detection system. A polarization beam splitter 30 splits theoptical path into two paths for a magneto-optical signal detectionsystem and a servo signal detection system. The light beam reflected bythe polarization beam splitter 30 is guided toward a photodetector 31via the condenser lens 6 and a cylindrical lens 19. The photodetector 31comprises a quadrant sensor, and obtains a focusing error signal and atracking error signal by the arrangement shown in FIG. 16 (not shown).Since the aperture 39 is not arranged between the objective lens 4 andthe condenser lens 6, a normal circular light spot is formed on thephotodetector 31.

The light beam transmitted through the polarization beam splitter 30passes through the aperture 39 of the present invention via a halfwaveplate 17, and some marginal rays in the radial and track directions aremasked. The aperture 39 is arranged in the vicinity of the pupil of thelight-receiving system, as described above. The light beam transmittedthrough or reflected by a polarization beam splitter 18 is guided ontophotodetectors 7-1 and 7-2.

Reproduction of a magneto-optical signal will be explained below withreference to FIG. 45. FIG. 45 illustrates a state wherein the light beamreflected by the polarization beam splitter 18 forms light spots 20-1and 20-2 on the photodetectors 7-1 and 7-2. Since marginal rays in theradial and track directions are masked by the aperture 39, a light spotshape shown in FIG. 41 is obtained. A magneto-optical signal 28 isdetected by differentially amplifying sum outputs from thephotodetectors 7-1 and 7-2 by a differential amplifier 25.

As in an optical system shown in FIG. 44, when the aperture 39 does notinfluence servo signals since independent detection systems are arrangedto detect a magneto-optical signal and servo signals, the aperture 39preferably has B1/A1=0.40 to 0.85. More preferably, the aperture 39 hasB1/A1=0.45 to 0.75. Also, the aperture 39 preferably has C1/A1=0.55 to0.90. More preferably, the aperture 39 has C1/A1=0.60 to 0.85. When suchan aperture is inserted in the vicinity of the pupil of thelight-receiving system, crosstalk components from an adjacent track andinformation from an adjacent mark are masked, and crosstalk componentsand jitter components due to an intersymbol interaction can be reduced.

As described above, when marginal rays in the radial and trackdirections of the light-receiving system are masked, crosstalkcomponents from an adjacent track and information from an adjacent markare masked, and crosstalk components and jitter components due to anintersymbol interaction can be reduced. As in the first to sixthembodiments, means for masking marginal rays in the track and radialdirections may be arranged in the vicinity of the pupil of thelight-receiving system or in the far field region sufficiently separatedfrom the focal point of the light-receiving system. The shape of theaperture 39 may be a rectangle or an ellipse. Also, marginal rays in thetrack and radial directions may be prevented from being received bymodifying the shape of the photodetectors. Furthermore, they can beapplied to an optical system which includes a single detection systemfor a magneto-optical signal and servo signals, and an optical systemwhich includes independent detection systems therefor. Note thatreduction of jitter components caused by a spherical abberation has beenparticularly exemplified. However, this embodiment is effective for acase wherein a coma or a defocus is generated.

When the present invention is adopted, easy adjustment of the apertureor photodetector is greatly improved as compared to the prior art inwhich a pinhole is inserted in the focal plane of the light-receivingsystem, and this arrangement is not easily influenced by a change intemperature or aging. Since the aperture is inserted in thelight-receiving system, a considerable decrease in light utilizationefficiency due to the insertion of an annual aperture can be preventedunlike in the prior art, and this arrangement is suitable for, e.g., amagneto-optical disc recording/reproduction apparatus which requireshigh power upon recording/erasing of information.

As described above, when an aperture for masking marginal rays, in theradial direction, of returned light from a disc is arranged in a regionsufficiently separated from the focal plane of an optical headlight-receiving system, e.g., in the vicinity of the pupil of thelight-receiving system, the influence of crosstalk from an adjacenttrack can be effectively reduced. In addition, the same effect can beexpected when marginal rays, in the radial direction of returned lightare masked by modifying the shape of the photodetector.

Also, when an aperture for masking marginal rays, in the trackdirection, of returned light from a disc is arranged in a regionsufficiently separated from the focal plane of an optical headlight-receiving system, e.g., in the vicinity of the pupil of thelight-receiving system, information from an adjacent mark can be masked,and jitter components due to an intersymbol interaction can be reduced.In addition, the same effect can be expected when marginal rays, in thetrack direction, of returned light are masked by modifying the shape ofthe photodetector.

The present invention can be widely applied to both an optical systemwhich includes a single detection system for detecting a magneto-opticalsystem and servo signals, and an optical system which includesindependent detection systems therefor. When the present invention isadopted, easy adjustment of the aperture or photodetector is greatlyimproved as compared to the prior art in which a pinhole is inserted inthe focal plane of the light-receiving system, and this arrangement isnot easily influenced by a change in temperature or aging. Since theaperture is inserted in the light-receiving system, a considerabledecrease in light utilization efficiency due to the insertion of anannular aperture can be prevented unlike in the prior art, and thisarrangement is suitable for, e.g., a magneto-optical discrecording/reproduction apparatus which requires high power uponrecording/erasing of information.

Since crosstalk components from an adjacent track or intersymbolinteraction factors from a mark adjacent in the track direction, whichare included in a side lobe portion, can be effectively reduced byinserting a simple aperture or by modifying the shape of thephotodetector, an optical head can have a compact structure. Inaddition, since easy adjustment of the aperture or photodetector is alsoimproved, cost can be reduced.

As described in the above embodiments, according to the presentinvention, when recording/reproduction of information or reproduction ofinformation is performed by irradiating a light beam from a light sourceas a fine light spot onto a predetermined track of an optical recordingmedium having a plurality of neighboring tracks, mask means for maskingmarginal rays, in a direction perpendicular to the track, of a returnedlight beam from the optical recording medium is arranged in the farfield region sufficiently separated from the focal plane of a detectionoptical system for detecting the returned light beam from the opticalrecording medium in an optical path of the detection optical system,thereby reducing information (crosstalk components from an adjacenttrack) reproduced from the adjacent track upon reproduction ofinformation from the predetermined track. The mask means for masking thelight beam preferably has an opening having a ratio of the width of theopening to the beam diameter of the returned light beam in the directionperpendicular to the track, which ratio falls within a range from 0.4 to0.9.

Also, as described in the above embodiments, according to the presentinvention, when recording/reproduction of information or reproduction ofinformation is performed by irradiating a light beam from a light sourceas a fine light spot onto a predetermined track of an optical recordingmedium, mask means for masking marginal rays, in the track direction, ofa returned light beam from the optical recording medium is arranged inthe far field region sufficiently separated from the focal plane of adetection optical system for detecting the returned light beam from theoptical recording medium in an optical path of the detection opticalsystem, thereby reducing information (intersymbol interaction)reproduced from an adjacent mark located on a single track uponreproduction of a predetermined track located on the track. The maskmeans for masking the light beam preferably has an opening having aratio of the width of the opening to the beam diameter of the returnedlight beam in the track direction, which ratio falls within a range from0.55 to 0.9.

The ratios of the aperture width to the beam diameter of the returnedlight were calculated under the following conditions. Upon evaluation ofcrosstalk components from an adjacent track, the wavelength of thesemiconductor laser was set to be λ=780 nm, the NA of the objective lenswas set to be 0.55, the track pitch was set to be 1.4 μm, the trackwidth of the recorded portion was set to be 0.9 μm, the mark length ofthe carrier was set to be 0.75 μm, and the mark length of crosstalkcomponents recorded on the adjacent track was set to be 3.0 μm. Uponevaluation of information (intersymbol interaction) reproduced from anadjacent mark on a single track, a mark edge recording method with aminimum mark length=0.75 μm and a 1-7 modulation method for symbols wereused.

However, it was found from more detailed simulations that the crosstalkamount from an adjacent track was an amount associated with the lightspot diameter determined based on the NA of the objective lens and thewavelength of the semiconductor laser in the optical head, and the trackpitch if neither a tilt of the disc substrate nor a substrate thicknesserror occurred (in other words, when the coma and the sphericalaberration were satisfactorily corrected). Thus, the ratio of theopening width to the beam diameter of the returned light includes theseparameters. It was also found that the crosstalk amount from an adjacenttrack was an amount associated with the wave aberration coefficient of acoma when, e.g., the tilt of the disc substrate was noticeable and acoma was generated. Similarly, it was found that the crosstalk amountfrom an adjacent track was an amount associated with the wave aberrationcoefficient of a spherical aberration when, e.g., the substratethickness error was noticeable and a spherical aberration was generated.

Furthermore, it was found that the intersymbol interaction amount was anamount associated with the light spot diameter and the minimum marklength, which were determined based on the NA of the objective lens andthe wavelength of the semiconductor laser in the optical head, ifneither a tilt of the disc substrate nor a substrate thickness erroroccurred. It was found that the intersymbol interaction amount was anamount associated with the wave aberration coefficient of a coma when,e.g., the tilt of the disc substrate was noticeable and a coma wasgenerated. Similarly, it was found that the intersymbol interactionamount was an amount associated with the wave aberration coefficient ofa spherical aberration when, e.g., the substrate thickness error wasnoticeable and a spherical aberration was generated.

According to the present invention in the embodiments to be describedbelow, the recording/reproduction of information or reproduction ofinformation is performed by irradiating a light beam from a light sourceas a fine light spot onto a predetermined track of an optical recordingmedium having a plurality of neighboring tracks, mask means for maskingmarginal rays, in a direction perpendicular to the track, of a returnedlight beam from the optical recording medium is arranged in the farfield region sufficiently separated from the focal plane of a detectionoptical system for detecting the returned light beam from the opticalrecording medium in an optical path of the detection optical system,thereby reducing information (crosstalk components from an adjacenttrack) reproduced from the adjacent track upon reproduction ofinformation from the predetermined track. The mask means is set tosatisfy the following relations.

When a reproduction signal of information and a tracking servo signalare generated from a single photodetector in a state wherein the comaand the spherical aberration are satisfactorily corrected, the optimalratio (B1/A1) of the aperture width B1 of the mask means to the beamdiameter A1 of the returned light beam satisfies:

    0.74-0.21·(d1/p)<B1/A1<1.09-0.21·(d1/p)

where d1 is the 1/e² diameter (defined by 1/e² of the central intensity)of the light spot of the optical recording medium in the directionperpendicular to the track and p is the track pitch of the opticalrecording medium.

When a reproduction signal of information and a tracking servo signalare generated from independent photodetectors in a state wherein thecoma and the spherical aberration are satisfactorily corrected, theratio (B1/A1) satisfies:

    0.64-0.21·(d1/p)<B1/A1<1.09-0.21·(d1/p)

When a reproduction signal of information and a tracking servo signalare generated from a single photodetector in a state wherein the coma isdominant, the ratio (B1/A1) satisfies:

    0.74-0.21·(d1/p)-0.25·W31<B1/A1<1.09-0.21·(d1/p)-0.25·W31

where W31 is the wave aberration coefficient of the coma.

When a reproduction signal of information and a tracking servo signalare generated from independent photodetectors in a state wherein thecoma is dominant, the ratio (B1/A1) satisfies:

    0.64-0.21·(d1/p)-0.25·W31<B1/A1<1.09-0.21·(d1/p)-0.25·W31

where W31 is the wave aberration coefficient of the coma.

When a reproduction signal of information and a tracking servo signalare generated from a single photodetector in a state wherein thespherical aberration is dominant, the ratio (B1/A1) satisfies:

    0.74-0.21·(d1/p)-0.26·W40.sup.2 <B1/A1<1.09-0.21·(d1/p)-0.26·W40.sup.2

where W40 is the wave aberration coefficient of the sphericalaberration.

When a reproduction signal of information and a tracking servo signalare generated from independent photodetectors in a state wherein thespherical aberration is dominant, the ratio (B1/A1) satisfies:

    0.64-0.21·(d1/p)-0.26·W40.sup.2 <B1/A1<1.09-0.21·(d1/p)-0.26·W40.sup.2

where W40 is the wave aberration coefficient of the sphericalaberration.

According to the present invention in the embodiments to be describedbelow, when recording/reproduction of information or reproduction ofinformation is performed by irradiating a light beam from a light sourceas a fine light spot onto a predetermined track of an optical recordingmedium, mask means for masking marginal rays, in the track direction, ofa returned light beam from the optical recording medium is arranged inthe far field region sufficiently separated from the focal plane of adetection optical system for detecting the returned light beam from theoptical recording medium in an optical path of the detection opticalsystem, thereby reducing information (intersymbol interaction)reproduced from an adjacent mark on a single track upon reproduction ofa predetermined mark located on the track. The mask means is set tosatisfy the following relations.

When the coma and the spherical aberration are satisfactorily corrected,the optimal ratio (B2/A2) of the aperture width B2 of the mask means tothe beam diameter A2 of the returned light beam satisfies:

    0.77-0.1·(d2/m)<B2/A2<1.07-0.1·(d2/m)

where d2 is the 1/e² diameter (defined by 1/e² of the central intensity)of the light spot of the optical recording medium in the track directionand m is the minimum mark length on a disc.

When the coma is dominant, the ratio (B2/A2) satisfies:

    0.77-0.1·(d2/m)-0.12·W31<B2/A2<1.07-0.1·(d2/m)-0.12·W31

where W31 is the wave aberration coefficient of the coma.

When the spherical aberration is dominant, the ratio (B2/A2) satisfies:

    0.77-0.1·(d2/m)-0.12·W40.sup.2 <B2/A2<1.07-0.1·(d2/m)-0.12·W40.sup.2

where W40 is the wave aberration coefficient of the sphericalaberration.

Still other embodiments of the present invention will be describedhereinafter.

The arrangement according to the ninth embodiment of the presentinvention will be described below with reference to FIGS. 46, 47, and 7.FIG. 46 is a sectional view, in the radial direction, of amagneto-optical disc 5 in an optical system of the present invention,and FIG. 47 is a sectional view, in the track direction, of the disc 5.FIGS. 46 and 47 particularly illustrate only a detection optical systemfor detecting returned light from a disc. The shape of an aperture 12 isthe same as that shown in FIG. 7 described above. Referring to FIGS. 46and 47, an objective lens 4 forms a fine light spot 9 on the informationrecording surface of the magneto-optical disc 5. Assume that the lightspot 9 is imaged in an ideal state, and the coma and the sphericalaberration are satisfactorily corrected.

Referring to FIG. 46, an arrow 13 indicates the radial direction, and aplurality of tracks are aligned in the direction of the arrow 13. A1 inFIG. 46 represents the width of a light beam of the most marginalportion of an irradiation optical system for forming and irradiating alight spot onto a disc (to be referred to as a light projection systemhereinafter), and this width A1 is determined by an aperture 4' of anobjective lens. The width A1 represents the NA of the light projectionsystem of the objective lens. FIG. 46 also illustrates, beside thelight-receiving system, a state wherein a side lobe due to the influenceof the aperture 4' of the objective lens is generated in the light spot9. The light spot is illustrated in a state observed in the direction ofits rays, and its a-a' section corresponds to the radial direction.

FIG. 48 shows the light intensity distribution of the light spot 9 inthe a-a' section. Referring to FIG. 48, the abscissa represents thelight spot radius, and the ordinate (left side) represents the relativeintensity when the central intensity of the light spot is 100%. Theordinate (right side) has a scale 10 times that of the ordinate on theleft side, and represents, in detail, the state of the side lobe. Thislight spot is obtained by focusing a light beam having a wavelengthλ=780 nm using an objective lens having NA=0.55. As can be seen fromFIG. 48, a side lobe having an intensity as high as about 2% of thecentral intensity is generated.

When information on a given track is reproduced in this state, the sidelobe due to the influence of the aperture of the objective lensundesirably reproduces information on an adjacent track, and theinformation reproduced from an adjacent track is found in a reproductionsignal as crosstalk components. Such a problem of the crosstalkcomponents from an adjacent track is unavoidable in a conventionaloptical system, and seriously disturbs an increase in density.

Returned light from the magneto-optical disc 5 is collimated via anobjective lens 4, and is guided toward a photodetector 7 by a condenserlens 6. The aperture 12 is arranged between the objective lens 4 and thecondenser lens 6, i.e., in the vicinity of the pupil of thelight-receiving system, and masks marginal rays in the radial direction,so that these rays do not reach the photodetector 7. B1 in FIG. 46represents the width of a light beam of the most marginal portion of thelight-receiving system, and the width B1 is determined by the aperture12. The width B1 represents the NA of the light-receiving system of theobjective lens.

FIG. 46 illustrates rays which pass various height positions of thepupil diameter, and of these rays, hatched rays which are masked by theaperture 12 form a side lobe in the light spot on the disc under theinfluence of an aperture 4' of the objective lens 4. Therefore, most ofthe crosstalk components are included in these marginal rays, andcrosstalk components can be reduced by masking these rays.

The side lobe due to the influence of the aperture 4' and marginal raysin the pupil of the light-receiving system normally have a goodcorrespondence therebetween although they do not have a strictone-to-one correspondence therebetween unlike that between the lightspot on the disc and the light spot of the light-receiving system shownin FIG. 4.

Similarly, FIG. 47 illustrates, beside the light-receiving system, astate wherein a side lobe due to the influence of the aperture 4' of theobjective lens 4 is generated in the light spot 9. The b-b' section ofthe light spot corresponds to the track direction. Returned light fromthe magneto-optical disc 5 is collimated via the objective lens 4, andis guided toward the photodetector 7 by the condenser lens 6.

The aperture 12 has a dimension in the track direction larger than thebeam diameter A1 so as not to mask marginal rays in the track direction.This is to guide modulated components of a reproduction signal from thedisc to the photodetector as much as possible since these components arespatially distributed in the track direction in the pupil of thelight-receiving system.

FIG. 7 is a front view of the aperture 12 used in the optical systemshown in FIGS. 46 and 47. The dimension, in the radial direction, of theaperture is B1 (B1<A1), and the dimension, in the track direction, ofthe aperture is C1 (C1>A1). If the objective lens 4 has a focal lengthfo=3 mm and NA=0.55, the beam diameter A=3.3 mm. As will be describedlater, since B is selected to be about several mm, easy adjustment ofthe aperture is greatly improved as compared to the prior art in which apinhole having a diameter of 15 μm is inserted in the focal plane of thelight-receiving system, and the aperture is not easily influenced by achange in temperature or aging. Since the aperture 12 is inserted in thelight-receiving system, a considerable decrease in light utilizationefficiency due to the insertion of an annular aperture can be preventedunlike in the prior art, and this arrangement is suitable for, e.g., amagneto-optical disc recording/reproduction apparatus which requireshigh power upon recording/erasing of information.

FIGS. 49 and 50 show the computer simulation results of the reductioneffect of crosstalk components from an adjacent track, which areobtained by changing the NA of the objective lens, the wavelength of thesemiconductor laser, and the track pitch in the optical system accordingto the ninth embodiment of the present invention.

FIG. 49 shows the calculation results obtained using two combinations ofoptical heads and discs.

More specifically, FIG. 49 shows, as the first example, results obtainedby using an optical head having a wavelength λ=780 nm and an NA=0.55 ofthe objective lens (to be referred to as optical head 1 hereinafter),and a disc having a track pitch p=1.4 μm, a mark length=0.75 μm of acarrier (reproduction signal), and a mark length=3.0 μm of crosstalkcomponents recorded on an adjacent track (to be referred to as disc 1hereinafter) (mark ∘ in FIG. 49).

Also, FIG. 49 shows, as the second example, results obtained by using anoptical head having a wavelength λ=680 nm and an NA=0.60 of theobjective lens (to be referred to as optical head 2 hereinafter), and adisc having a track pitch p=1.1 μm, a mark length=0.64 μm of a carrier(reproduction signal), and a mark length=2.6 μm of crosstalk componentsrecorded on an adjacent track (to be referred to as disc 2 hereinafter)(mark □ in FIG. 49). In both the examples, a change in crosstalk amountfrom the adjacent track is calculated by changing the width B, in theradial direction, of the aperture 12. The abscissa represents the ratioof the width B1, in the radial direction, of the aperture 12 to the beamdiameter A.

The ordinate represents an amount in units of dB, which is obtained bynormalizing the crosstalk amount mixed from the adjacent track uponreproduction with the carrier. As can be seen from FIG. 49, in both thecombinations, the crosstalk amounts from the adjacent tracks are almostequal to each other, and the crosstalk amounts are similarly reduced asB1/A1 becomes smaller.

FIG. 50 shows the calculation results obtained using a total of sixcombinations of optical heads and discs by adding two discs to those inFIG. 49. In FIG. 50, the first example is indicated by a mark □, and thesecond example is indicated by a mark ∘.

Assume, as disc 3, a disc which has a track pitch p=1.6 μm, a marklength=0.78 μm of a carrier, and a mark length=3.1 μm of crosstalkcomponents recorded on the adjacent track. Assume, as disc 4, a discwhich has a track pitch p=0.8 μm, a mark length=0.47 μm of a carrier,and a mark length=1.9 μm of crosstalk components recorded on theadjacent track.

FIG. 50 shows, as the third example, the calculation results obtainedusing a combination of optical head 1 and disc 2 (mark ∘ in FIG. 50).FIG. 50 shows, as the fourth example, the calculation results obtainedusing a combination of optical head 1 and disc 3 (mark Δ in FIG. 50).FIG. 50 shows, as the fifth example, the calculation results obtainedusing a combination of optical head 2 and disc 4 (mark × in FIG. 50).FIG. 50 shows, as the sixth example, the calculation results obtainedusing a combination of optical head 2 and disc 1 (mark □ in FIG. 50).

In these examples, a change in crosstalk amount from an adjacent trackis calculated by changing the width B1, in the radial direction, of theaperture 12.

The abscissa represents the ratio of the width B1, in the radialdirection, of the aperture 12 to the beam diameter A1. The ordinaterepresents the crosstalk amount from an adjacent track in units of dB,which amount is normalized with a crosstalk amount when no aperture isarranged (B1/A1=1) in the respective combinations.

In order to examine the relationship between the reduction effect ofcrosstalk components from an adjacent track when the aperture is usedand the combinations of the optical heads and discs, Tables 1 and 2below summarize the light spot diameter determined by the NA of theobjective lens and the wavelength of the semiconductor laser of theoptical head, and the track pitch of the disc.

                  TABLE 1                                                         ______________________________________                                        (Spot Diameter of Optical Head)                                                            Optical Head 1                                                                         Optical Head 2                                          ______________________________________                                         (nm)          780        680                                                 Objective Lens NA                                                                            0.55       0.60                                                Spot Diameter (μm)                                                                        1.26       0.99                                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        (Spot Diameter and Track Pitch)                                                           dl/p                                                                            Optical Head 1                                                                           Optical Head 2                                                      = 780 nm   = 680 nm                                            Track Pitch (μm)                                                                         NA = 0.55  NA = 0.60                                            ______________________________________                                        0.8                      1.24                                                 1.1           1.15       0.90                                                 1.4           0.90       0.71                                                 1.6           0.79                                                            ______________________________________                                    

Table 1 shows the 1/e² diameters (defined by 1/e² of the centralintensity) of the light spot in the direction perpendicular to the trackformed by optical heads 1 and 2. The light spot diameter of optical head1 used in the simulations is 1.26 μm, and that of optical head 2 is 0.99μm. The relationship between the 1/e² diameter of the light spot and themaximum peak intensity of a side lobe is almost constant independentlyof optical heads, and corresponds to a position of a radius of about 1.2μm (diameter of 2.4 μm) in, e.g., optical head 1, as shown in FIG. 48.

Table 2 shows the relationship associated with the ratio d1/p of the1/e² diameter of the light spot shown in Table 1 to the track pitch p ofthe disc. For example, as shown in FIG. 49, in the combination ofoptical head 1 and disc 1 (first example) and the combination of opticalhead 2 and disc 2 (second example), the crosstalk amounts from adjacenttracks are almost equal to each other, and the crosstalk amounts aresimilarly reduced as B1/A1 becomes smaller. In these combinations, thevalues d1/p are equal to each other, i.e., 0.90.

More specifically, as can be seen from FIG. 49 and Table 2, when thecoma and the spherical aberration are satisfactorily corrected, theaperture ratio (B1/A1) for effectively reducing the crosstalk amountfrom an adjacent track is an amount associated with the light spotdiameter determined by the NA of the objective lens and the wavelengthof the semiconductor laser in the optical head, and the track pitch ofthe disc. It can be estimated from this fact that an optimal ratio ofthe aperture width in the radial direction to the beam diameter of thereturn light includes these parameters.

The aperture ratio (B1/A1) which decreases the crosstalk amount from anadjacent track by a predetermined amount (e.g., 10 dB) from the amountobtained when no aperture is arranged (B1/A1=1) in FIG. 50 will beexamined below in association with the combinations of optical heads anddiscs. FIG. 51 shows the examination results between the aperture ratioand d1/p. In FIG. 51, the abscissa represents d1/p, and the ordinaterepresents the aperture ratio (B1/A1) which decreases the crosstalkamount by 10 dB from the amount obtained when no aperture is arranged.As can be seen from FIG. 51, the combinations of the first to sixthexamples are distributed almost on a straight line. An approximation ofthe straight line is as follows:

    B1/A1=0.87-0.21·(d1/p)                            (6)

When a decrease in crosstalk amount as compared to the amount obtainedwhen no aperture is arranged is appropriately selected to be a valueother than 10 dB, an equation based on equation (6) is established:

    B1/A1=K-0.21·(d1/p)                               (6')

where K is a constant.

Since the optical system shown in FIG. 15 includes a single detectionsystem for a magneto-optical signal and a tracking servo signal, somerays in the radial direction are masked by the aperture 12, and theamplitude of the tracking signal is lowered. As has been described abovewith reference to FIG. 17, as B1/A1 becomes smaller, the amplitude ofthe tracking signal is lowered quadratically. For example, whenB1/A1=0.7, the amplitude of the tracking signal becomes about 70% of thevalue obtained when no aperture is arranged. This suggests that rayswhich are masked to reduce crosstalk components from an adjacent trackinclude components modulated by a groove crossing signal. Therelationship between the aperture ratio and the decrease in amplitude ofthe tracking signal is almost constant independently of the combinationsof optical heads and discs if d1/p remains the same. When d1/p becomeslarge, the amplitude of the tracking error signal obtained when noaperture is arranged becomes small. In addition, when the aperture ratiobecomes small, the amplitude of the tracking error signal tends todecrease immediately.

Based on the above description, setting of an optimal aperture ratiocorresponding to d1/p will be examined below. From equation (6), whenthe track pitch is small as compared to the 1/e² diameter of the lightspot, i.e., when d1/p is large, if the crosstalk amount from an adjacenttrack is to be decreased by a predetermined amount as compared to theamount obtained when no aperture is arranged, an aperture having asmaller aperture ratio must be inserted in the optical path. Meanwhile,a decrease in amplitude of the tracking signal caused by insertion ofthe aperture poses a problem.

In the experiments using the combination of the first example(d1/p=0.9), the aperture 12 preferably had B1/A1=0.55 to 0.90. Morepreferably, the aperture 12 has B1/A1=0.60 to 0.80. When such anaperture was inserted in the vicinity of the pupil of thelight-receiving system, crosstalk components from an adjacent trackcould be effectively reduced, and a decrease in amplitude of thetracking signal fell within an allowable range. When this condition issubstituted in equation (6'), the constant K preferably satisfies:

    0.74<K<1.09

More preferably, the constant K satisfies:

    0.79<K<0.99

From these conditions, B1/A1 preferably satisfies:

    0.74 0.21·(d1/p)<B1/A1<1.09-0.21·(d1/p)  (7)

More preferably, B1/A1 satisfies:

    0.79-0.21·(d1/p)<B1/A1<0.99-0.21·(d1/p)  (8)

for 0<B1/A1<1.

Setting of an optimal aperture ratio corresponding to d1/p whenindependent detection systems are arranged to detect a magneto-opticalsignal and a tracking servo signal as in the optical system shown inFIG. 19 will be examined below. From equation (6), when the track pitchis small as compared to the 1/e² diameter of the light spot, i.e., whend1/p is large, if the crosstalk amount from an adjacent track is to bedecreased by a predetermined amount as compared to the amount obtainedwhen no aperture is arranged, an aperture having a smaller apertureratio must be inserted in the optical path. Although the aperture 12does not influence the servo signal in this case, deterioration of theC/N (carrier to noise) ratio due to a decrease in carrier level causedby insertion of the aperture must be taken into consideration. Therelationship between the aperture ratio and the carrier level is almostconstant independently of the combinations of optical heads and discs ifd1/p remains the same. When d1/p becomes large, the carrier level tendsto drop. In addition, when the wavelength is shortened, thephotoelectric conversion efficiency of the photodetector is lowered, andthe carrier level tends to become small.

In the experiments using the combination of the first example(d1/p=0.9), the aperture 12 preferably had B1/A1=0.45 to 0.90. Morepreferably, the aperture 12 has B1/A1=0.50 to 0.80. When this conditionis substituted in equation (6'), the constant K preferably satisfies:

    0.64<K<1.09

More preferably, the constant K satisfies:

    0.69<K<0.99

From these conditions, B1/A1 preferably satisfies:

    0.64-0.21·(d1/p)<B1/A1<1.09-0.21·(d1/p)  (9)

More preferably, B1/A1 satisfies:

    0.69-0.21·(d1/p)<B1/A1<0.99-0.21·(d1/p)  (10)

When such an aperture is inserted in the vicinity of the pupil of thelight-receiving system, crosstalk components from an adjacent track canbe effectively reduced. Note that 0<B1/A1<1.

Since the optical system shown in FIG. 23 includes independent detectionsystems for a magneto-optical signal and a tracking servo signal, thedimension, in the radial direction, of the light-receiving portion ofthe photodetector is represented by B1 in place of the aperture, andB1/A1 preferably satisfies inequality (9). More preferably, B1/A1satisfies inequality (10). When a photodetector with such a dimension isused, crosstalk components from an adjacent track can be effectivelyreduced.

FIGS. 52 to 55 show the computer simulation results of the reductioneffect of crosstalk components from an adjacent track, which areobtained by changing the NA of the objective lens of the optical head,the wavelength of the semiconductor laser, and the track pitch in theoptical system according to the first embodiment of the presentinvention shown in FIGS. 5 and 6.

FIG. 52 shows the calculation results obtained using combinations ofoptical head 1 (the wavelength λ=780 nm, the NA=0.55 of the objectivelens) with discs 1, 2, and 3, which are tilted in the radial direction.These discs 1, 2, and 3 respectively have a track pitch p=1.4 μm, 1.1μm, and 1.6 μm. Each disc is tilted by 6.5 mrad. (milliradians) and 3.9mrad. For each of the combinations, a change in crosstalk amount from anadjacent track is calculated by changing the width B1, in the radialdirection, of the aperture 12.

The abscissa represents the ratio of the width B1, in the radialdirection, of the aperture 12 to the beam diameter A1. The ordinaterepresents an amount in units of dB, which is obtained by normalizing acrosstalk amount mixed from an adjacent track upon reproduction with thecarrier. As can be seen from FIG. 52, the crosstalk amount becomeslarger as the tilt of the disc is larger and as the track pitch issmaller, and the crosstalk amount is gradually reduced as B1/A1 becomessmaller.

FIG. 53 shows, for the purpose of comparison, the states of decreases incrosstalk amount corresponding to the respective tilts of the discsobtained when B1/A1 is changed in FIG. 52. The abscissa represents thewidth B1, in the radial direction, of the aperture 12 to the beamdiameter A1. The ordinate represents the crosstalk amount from anadjacent track, i.e., an amount in units of dB, which is normalized witha crosstalk amount obtained when no aperture is arranged (B1/A1=1) inthe respective combinations. As can be seen from FIG. 53, when B1/A1 isdecreased, the crosstalk amount from an adjacent track is reduced moreslowly as the tilt of the disc becomes larger and as the track pitch issmaller.

FIG. 54 shows the calculation results obtained using combinations ofoptical head 2 (the wavelength λ=680 nm, the NA=0.60 of the objectivelens) with discs 1, 2, and 4, which are tilted in the radial direction.These discs 1, 2, and 4 respectively have a track pitch p=1.4 μm, 1.1μm, and 0.8 μm. Each disc is tilted by 4.4 mrad. and 2.6 mrad. For eachof the combinations, a change in crosstalk amount from an adjacent trackis calculated by changing the width B1, in the radial direction, of theaperture 12.

The abscissa represents the ratio of the width B1, in the radialdirection, of the aperture 12 to the beam diameter A1. The ordinaterepresents an amount in units of dB, which is obtained by normalizing acrosstalk amount mixed from an adjacent track upon reproduction with thecarrier. As can be seen from FIG. 54, the crosstalk amount becomeslarger as the tilt of the disc is larger and as the track pitch issmaller, and the crosstalk amount is gradually reduced as B1/A1 becomessmaller, as in FIG. 52.

FIG. 55 shows, for the purpose of comparison, the states of decreases incrosstalk amount corresponding to the respective tilts of the discsobtained when B1/A1 is changed in FIG. 54. The abscissa represents thewidth B1, in the radial direction, of the aperture 12 to the beamdiameter A1. The ordinate represents the crosstalk amount from anadjacent track, i.e., an amount in units of dB, which is normalized withthe crosstalk amount obtained when no aperture is arranged (B1/A1=1) inthe respective combinations. As can be seen from FIG. 55, when B1/A1 ischanged, the crosstalk amount from an adjacent track is reduced moreslowly as the tilt of the disc becomes larger and as the track pitch issmaller as in FIG. 53.

The relationship between combinations of optical heads and discs and thereduction effect upon reduction of crosstalk amounts from an adjacenttrack using an aperture in an optical disc optical system which suffersa coma due to a tilt of the disc will be examined below. Tables 3 and 4below summarize the light spot diameter and the track pitch of the discwhen the disc is tilted.

                  TABLE 3                                                         ______________________________________                                        (Spot Diameter of Optical Head)                                               Spot Diameter (μm)                                                                        Optical Head 1                                                                           Optical Head 2                                      ______________________________________                                         (nm)          780        680                                                 Objective Lens NA                                                                            0.55       0.60                                                Disc Tilt (mrad.)                                                             2.6                       1.02                                                3.9            1.28                                                           4.4                       1.05                                                6.5            1.30                                                           ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        (Spot Diameter and Track Pitch)                                                            dl/p                                                                            Optical Head 1                                                                           Optical Head 2                                                      = 780 nm   = 680 nm                                           Track Pitch (μm)                                                                          NA = 0.55  NA = 0.60                                           Disc Tilt (mrad.)                                                                            3.9    6.5     2.6   4.4                                       ______________________________________                                        0.8                           1.28  1.31                                      1.1            1.16   1.18    0.93  0.95                                      1.4            0.91   0.93    0.73  0.75                                      1.6            0.80   0.81                                                    ______________________________________                                    

Table 3 shows the 1/e² diameter (radial direction) of the light spotformed by optical heads 1 and 2 when the disc is tilted in the radialdirection. As compared to a case wherein the disc is not tilted, thespot diameter increases by several %, and a crescent-shaped side lobe isgenerated, as shown in FIG. 5. On the other hand, the spot diameter inthe track direction is almost not changed. Table 4 shows the ratio d1/pof the 1/e² diameter d of the light spot shown in Table 3 to the trackpitch p.

When a coma is generated due to, e.g., a tilt of the disc, it isconsidered that the side lobe largely contributes to the crosstalkamount from an adjacent track in addition to d1/p. Thus, a coma upongeneration of a disc tilt in each optical head will be discussed below.When the disc tilt is relatively small, e.g., less than 1°, a waveaberration coefficient W31 of the 3rd order coma generated by the tiltis given by:

    W31=-t/2·n.sup.2 (n.sup.2 -1)sin θ cos θ/(n.sup.2 -sin.sup.2 θ).sup.5/2 ·(NA).sup.3          (11)

where t: the thickness of the disc substrate, n: the refractive index ofthe disc substrate, θ: the tilt of the disc substrate with respect tothe objective lens, and NA: the numerical aperture (light projectionsystem) of the objective lens.

As can be understood from equation (11), when the NA of the objectivelens varies, different comas are generated even when the disc tiltremains the same. For example, a coma generated by optical head 2 havingNA=0.60, λ=680 nm is about 1.5 times that generated by optical head 1having NA=0.55, λ=780 nm. In optical head 1, when the disc is tilted at3.9 mrad., W31=0.188λ; when the disc is tilted at 6.5 mrad., W31=0.313λ.In optical head 2, when the disc is tilted at 2.6 mrad. and 4.4 mrad.,the same comas as described above are generated.

FIGS. 53 and 55, Table 4, and equation (11) reveal that the apertureratio (B1/A1) for effectively reducing the crosstalk amount from anadjacent track upon generation of a coma is an amount associated withthe light spot diameter, the track pitch of the disc, and the comacoefficient W31. It can be estimated that an optimal ratio of theaperture width in the track direction to the beam diameter of returnedlight includes these parameters.

The aperture ratio (B1/A1) which decreases the crosstalk amount from anadjacent track by a predetermined amount (e.g., 10 dB) from the amountobtained when no aperture is arranged (B1/A1=1) in FIGS. 53 and 55 willbe examined below in association with the combinations of optical headsand discs. FIG. 56 shows the examination results of the relationshipamong B1/A1, d1/p, and the coma coefficient W31.

In FIG. 56, the abscissa represents d1/p, and the ordinate representsthe aperture ratio (B1/A1) which decreases the crosstalk amount by 10 dBfrom the amount obtained when no aperture is arranged. As can be seenfrom FIG. 56, the combinations of the first to sixth examples aredistributed almost on a straight line. An approximation of the straightline is as follows:

When W31=0 (when the disc is not tilted), from equation (6), we have:

    B1/A1=0.87-0.21·(d1/p)

When W31=0.188λ, we have:

    B1/A1=0.823-0.21·(d1/p)                           (12)

When W31=0.313λ, we have:

    B1/A1=0.792-0.21·(d1/p)                           (13)

As can be seen from these equations, as the coma increases, B1/A1 mustbe decreased in proportion to the coma coefficient so as to decrease thecrosstalk amount from an adjacent track by the predetermined amount.From equations (6), (12), and (13), when W31 is introduced in theseequations, we have:

    B1/A1=0.87-0.21·(d1/p)-0.25·W31          (14)

When a decrease in crosstalk amount as compared to the amount obtainedwhen no aperture is arranged is appropriately selected to be a valueother than 10 dB, an equation based on equation (14) is established:

    B1/A1=K-0.21·(d1/p)-0.25·W31             (14')

where K is a constant.

An optimal ratio of the aperture width in the radial direction to thebeam diameter of a returned light beam upon generation of a comaincludes parameters in equation (14'). Note that the disc tilt has beenexemplified as a factor of generation of a coma. Even when a coma isgenerated due to another factor, if the factor is expressed by the waveaberration coefficient W31, the same discussion as above applies.

The influence of crosstalk components from an adjacent track can beeffectively reduced by inserting the aperture 12 given by equation (14')in an optical head optical system which suffers a coma due to, e.g., adisc tilt.

As in the ninth embodiment of the present invention, a magneto-opticalhead optical system which includes the aperture 12 shown in FIGS. 15 and16 will be examined below. Since the optical system shown in FIG. 15includes a single detection system for a magneto-optical signal and atracking servo signal, some rays in the radial direction are masked bythe aperture 12, and the amplitude of the tracking signal is undesirablylowered. This suggests that rays which are masked to reduce crosstalkcomponents from an adjacent track include components modulated by agroove crossing signal.

The relationship between the aperture ratio and the decrease inamplitude of the tracking signal is almost constant independently of thecombinations of optical heads and discs if d1/p remains the same. Whend1/p becomes large, the amplitude of the tracking error signal obtainedwhen no aperture is arranged becomes small. In addition, when theaperture ratio becomes small, the amplitude of the tracking error signaltends to decrease immediately (see FIG. 17).

Based on the above description, setting of an optimal aperture ratiocorresponding to d1/p will be examined below. From equation (14'), whenthe track pitch is small as compared to the 1/e² diameter of the lightspot, i.e., when d1/p is large, if the crosstalk amount from an adjacenttrack is to be decreased by a predetermined amount as compared to theamount obtained when no aperture is arranged, an aperture having asmaller aperture ratio must be inserted in the optical path. When thedisc tilt is not strictly managed, a large coma may be generated. Forthis reason, an aperture having a still smaller aperture ratio must beinserted in the optical path. Meanwhile, a decrease in amplitude of thetracking signal caused by insertion of the aperture poses a problem.

In the experiments using the combination of the first example (d1/p=0.9)with a disc tilt=4 mrad. (W31=0.2λ or equivalent), the aperture 12preferably had B1/A1=0.50 to 0.85. More preferably, the aperture 12 hasB1/A1=0.55 to 0.75. When such an aperture was inserted in the vicinityof the pupil of the light-receiving system, crosstalk components from anadjacent track could be effectively reduced, and a decrease in amplitudeof the tracking signal fell within an allowable range.

When this condition is substituted in equation (14'), the constant Kpreferably satisfies:

    0.74<K<1.09

More preferably, the constant K satisfies:

    0.79<K<0.99

From these conditions, B1/A1 preferably satisfies:

    0.74-0.21·(d1/p)-0.25·W31<B1/A1<1.09-0.21·(d1/p)-0.25·W31                                         (15)

More preferably, B1/A1 satisfies:

    0.79-0.21·(d1/p)-0.25·W31<B1/A1<0.99-0.21·(d1/p)-0.25·W31                                         (16)

for W31≠0, 0<B1/A1<1

When the optical system described above with reference to FIGS. 19 and20 suffers a coma due to, e.g., a disc tilt, the influence of crosstalkcomponents from an adjacent track can be effectively reduced byinserting the aperture 12 given by equation (14').

A magneto-optical head optical system including the aperture 12 shown inFIGS. 19 and 20 will be examined below. The optical system shown in FIG.19 includes independent detection systems for a magneto-optical signaland a tracking servo signal. From equation (14'), when the track pitchis small as compared to the 1/e² diameter of the light spot, i.e., whend1/p is large, if the crosstalk amount from an adjacent track is to bedecreased by a predetermined amount as compared to the amount obtainedwhen no aperture is arranged, an aperture having a smaller apertureratio must be inserted in the optical path.

When the disc tilt is not strictly managed, a large coma may begenerated. For this reason, an aperture having a still smaller apertureratio must be inserted in the optical path. Although the aperture 12does not influence the servo signal in this case, deterioration of theC/N (carrier to noise) ratio due to a decrease in carrier level causedby insertion of the aperture must be taken into consideration. Therelationship between the aperture ratio and the carrier level is almostconstant independently of the combinations of optical heads and discs ifd1/p remains the same. When d1/p becomes large, the carrier level tendsto drop. In addition, when the wavelength is shortened, thephotoelectric conversion efficiency of the photodetector is lowered, andthe carrier level tends to drop.

In the experiments using the combination of the first example(d1/p=0.9), the aperture 12 preferably had B1/A1=0.40 to 0.85. Morepreferably, the aperture 12 has B1/A1=0.45 to 0.75.

When this condition is substituted in equation (14'), the constant Kpreferably satisfies:

    0.64<K<1.09

More preferably, the constant K satisfies:

    0.69<K<0.99

From these conditions, B1/A1 preferably satisfies:

    0.64-0.21·(d1/p)-0.25·W31<B1/A1<1.09-0.21·(d1/p)-0.25·W31                                         (17)

More preferably, B1/A1 satisfies:

    0.69-0.21·(d1/p)-0.25·W31<B1/A1<0.99-0.21·(d1/p)-0.25·W31                                         (18)

for W31≈0, 0<B1/A1<1

Furthermore, as in the embodiment described above with reference toFIGS. 23 and 24, the influence of crosstalk components from an adjacenttrack can be effectively reduced by receiving a light beam reflected bythe disc using the photodetectors 7-1 and 7-2 each of which has alight-receiving portion given by equation (14') in an optical headoptical system which suffers a coma due to, e.g., a disc tilt (see FIGS.23 and 24).

Since the optical system shown in FIG. 23 includes independent detectionsystems for a magneto-optical signal and a tracking servo signal, thedimension, in the radial direction, of the light-receiving portion ofeach photodetector is represented by B1 in place of the aperture, andB1/A1 preferably satisfies inequality (17). More preferably, B1/A1satisfies inequality (18). When photodetectors with such dimensions areused, crosstalk components from an adjacent track can be effectivelyreduced.

FIGS. 57 to 60 show the computer simulation results of the reductioneffect of crosstalk components from an adjacent track, which areobtained by changing the NA of the objective lens of the optical head,the wavelength of the semiconductor laser, and the track pitch in theoptical system according to the first embodiment of the presentinvention described above with reference to FIGS. 11 and 12.

FIG. 57 shows the calculation results using the combination of opticalhead 1 (the wavelength λ=780 nm, the NA=0.55 of the objective lens) anddisc 1 (the track pitch p=1.4 μm) which suffers a disc substratethickness error. Substrate thickness errors given to the disc are ±50μm, ±75 μm, and ±100 μm. In each of these cases, a change in crosstalkamount from an adjacent track is calculated by changing the width B1, inthe radial direction, of the aperture 12. The abscissa represents theratio of the width B1, in the radial direction, of the aperture 12 tothe beam diameter A1. The ordinate represents an amount in units of dB,which is obtained by normalizing a crosstalk amount mixed from anadjacent track upon reproduction with the carrier. As can be seen fromFIG. 57, the crosstalk amount from an adjacent track increases as thesubstrate thickness error becomes larger and as the track pitch becomessmaller. As can also be seen from FIG. 57, the crosstalk amount isgradually reduced as the B1/A1 becomes smaller.

FIG. 58 shows, for the purpose of comparison, the states of decreases incrosstalk amount corresponding to the respective substrate thicknesserrors of the discs obtained when B1/A1 is changed in FIG. 57. Theabscissa represents the width B1, in the radial direction, of theaperture 12 to the beam diameter A1. The ordinate represents thecrosstalk amount from an adjacent track, i.e., an amount in units of dB,which is normalized with the crosstalk amount obtained when no apertureis arranged (B1/A1=1) in the respective combinations. As can be seenfrom FIG. 58, the decrease in crosstalk amount from an adjacent trackobtained when B1/A1 is decreased is reduced more slowly as the substratethickness error of the disc becomes larger and as the track pitchbecomes smaller.

FIG. 59 shows the calculation results obtained using the combination ofoptical head 2 (the wavelength λ=680 nm, the NA=0.60 of the objectivelens) and disc 2 (the track pitch p=1.1 μm) which suffers a discsubstrate thickness error. Substrate thickness errors given to the discare ±31 μm, ±46 μm, and ±62 μm. In each of these cases, a change incrosstalk amount from an adjacent track is calculated by changing thewidth B1, in the radial direction, of the aperture 12. The abscissarepresents the ratio of the width B1, in the radial direction, of theaperture 12 to the beam diameter A1. The ordinate represents an amountin units of dB, which is obtained by normalizing the crosstalk amountmixed from an adjacent track upon reproduction with the carrier. As canbe seen from FIG. 59, the crosstalk amount from an adjacent trackbecomes larger as the substrate thickness error becomes larger and asthe track pitch becomes smaller, and the crosstalk amount is graduallyreduced as B1/A1 becomes smaller, as in FIG. 57.

FIG. 60 shows, for the purpose of comparison, the states of decreases incrosstalk amount corresponding to the respective substrate thicknesserrors of the discs obtained when B1/A1 is changed in FIG. 59. Theabscissa represents the width B1, in the radial direction, of theaperture 12 to the beam diameter A1. The ordinate represents thecrosstalk amount from an adjacent track, i.e., an amount in units of dB,which is normalized with the crosstalk amount obtained when no apertureis arranged (B1/A1=1) in the respective combinations. As can be seenfrom FIG. 60, the decrease in crosstalk amount from an adjacent trackobtained when B1/A1 is decreased is reduced more slowly as the substratethickness error becomes larger and as the track pitch becomes smaller,as in FIG. 58. Although not shown, in combinations of optical head 1with discs 2 and 3, and optical head 2 with discs 1 and 4, the samesubstrate thickness errors as described above were given, and the samesimulations were performed.

The relationship between combinations of optical heads and discs and thereduction effect upon reduction of crosstalk amounts from an adjacenttrack using an aperture in an optical disc optical system which suffersa spherical aberration due to a substrate thickness error of the discwill be examined below. Tables 5 and 6 below summarize the light spotdiameter and the track pitch of the disc when a substrate thicknesserror occurs.

                  TABLE 5                                                         ______________________________________                                        (Spot Diameter of Optical Head)                                               Spot Diameter (μm)                                                                        Optical Head 1                                                                           Optical Head 2                                      ______________________________________                                         (nm)          780        680                                                 Objective Lens NA                                                                            0.55       0.60                                                Substrate                                                                     Thickness Error                                                               (μm)                                                                       31                        0.99                                                46                        0.99                                                50             1.26                                                           62                        1.00                                                75             1.26                                                           100            1.27                                                           ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        (Spot Diameter and Track Pitch)                                                            dl/p                                                                            Optical Head 1                                                                             Optical Head 2                                                    = 780 nm     = 680 nm                                         Track Pitch (μm)                                                                          NA = 0.55    NA = 0.60                                         ______________________________________                                        Substrate      50     75     100  31   46   62                                0.8                               1.24 1.24 1.25                              1.1            1.15   1.15   1.16 0.90 0.90 0.91                              1.4            0.90   0.90   0.91 0.71 0.71 0.72                              1.6            0.79   0.79   0.80                                             ______________________________________                                    

Table 5 shows the 1/e² diameter (radial direction) of the light spotformed by optical heads 1 and 2 when the disc suffers a substratethickness error. As compared to a case wherein the disc suffers nosubstrate thickness error, the spot diameter shows almost no changes,but a concentrical side lobe is generated, as shown in FIG. 11.

Table 6 shows the ratio d1/p of the 1/e² diameter d of the light spotshown in Table 5 to the track pitch p.

When a spherical aberration is generated due to, e.g., a substratethickness error of the disc, it is considered that the side lobe largelycontributes to the crosstalk amount from an adjacent track in additionto d1/p. Thus, a spherical aberration upon generation of a substratethickness error in each optical head will be discussed below. When thesubstrate thickness error is relatively small, e.g., less than 100 μm, awave aberration coefficient W40 of the 3rd order spherical aberrationgenerated by the error is given by:

    W40=(n.sup.2 -1)/8n.sup.3 ·(NA).sup.4 ·Δt(19)

where Δt: the substrate thickness error of the disc, n: the refractiveindex of the disc, and NA: the numerical aperture (light projectionsystem) of the objective lens. As can be understood from equation (19),when the NA of the objective lens varies, different sphericalaberrations are generated even when the substrate thickness errorremains the same. For example, a spherical aberration generated byoptical head 2 having NA=0.60 is about 1.4 times that generated byoptical head 1 having NA=0.55. At the same time, since the substratethickness error of the disc affects more severely in inverse proportionto the wavelength of a light source, optical head 2 generates aspherical aberration about 1.6 times that generated by optical head 1 inthis case.

In optical head 1, when a disc substrate thickness error of ±50 μm isgenerated, W40=0.28λ; when a disc substrate thickness error of ±75 μm isgenerated, W40=0.41λ; and when a disc substrate thickness error of ±100μm is generated, W40=0.55λ. In optical head 2, when disc substratethickness errors of ±31 μm, ±46 μm, and ±62 μm are generated, the samespherical aberrations as described above are generated.

FIGS. 58 and 60, the calculation results based on other combinations,Table 6, and equation (19) reveal that the aperture ratio (B1/A1) foreffectively reducing the crosstalk amount from an adjacent track upongeneration of a spherical aberration is an amount associated with thelight spot diameter, the track pitch of the disc, and the sphericalaberration coefficient W40. It can be estimated that an optimal ratio ofthe aperture width in the radial direction to the beam diameter ofreturned light includes these parameters.

The aperture ratio (B1/A1) which decreases the crosstalk amount from anadjacent track by a predetermined amount (e.g., 10 dB) from the amountobtained when no aperture is arranged (B1/A1=1) will be examined belowin association with the combinations of optical heads and discs. FIG. 61shows the examination results of the relationship among B1/A1, d1/p, andthe spherical aberration coefficient W40. In FIG. 61, the abscissarepresents d1/p, and the ordinate represents the aperture ratio (B1/A1)which decreases the crosstalk amount by 10 dB from the amount obtainedwhen no aperture is arranged. As can be seen from FIG. 61, thecombinations of the first to sixth examples are distributed almost on astraight line. An approximation of the straight line is as follows:

When W40=0 (when no disc substrate thickness error is generated), fromequation (6), we have:

    B1/A1=0.87-0.21·(d1/p)

When W40=0.28λ, we have:

    B1/A1=0.85-0.21·(d1/p)                            (20)

When W40=0.41λ, we have:

    B1/A1=0.825-0.21·(d1/p)                           (21)

When W40=0.55λ, we have:

    B1/A1=0.79-0.21·(d1/p)                            (22)

When the spherical aberration increases, B1/A1 must be decreased inproportion to a square of the spherical aberration coefficient so as todecrease the crosstalk amount from an adjacent track by thepredetermined amount. From equations (6), and (20) to (22), when W40 isintroduced in these equations, we have:

    B1/A1=0.87-0.21·(d1/p)-0.26·W40.sup.2    (23)

When a decrease in crosstalk amount as compared to the amount obtainedwhen no aperture is arranged is appropriately selected to be a valueother than 10 dB, an equation based on equation (23) is established:

    B1/A1=K-0.21·(d1/p)-0.26·W40.sup.2       (23')

where K is a constant. An optimal ratio of the aperture width in thetrack direction to the beam diameter of a returned light beam upongeneration of a spherical aberration includes parameters in equation(23'). Note that the disc substrate thickness error has been exemplifiedas a factor of generation of a spherical aberration. Even when aspherical aberration is generated due to another factor, if the factoris expressed by the wave aberration coefficient W40, the same discussionas above applies.

The influence of crosstalk components from an adjacent track can beeffectively reduced by inserting the aperture 12 given by equation (23')in an optical head optical system which suffers a spherical aberrationdue to a disc substrate thickness error. As in the ninth embodiment ofthe present invention, a magneto-optical head optical system whichincludes the aperture 12 shown in FIGS. 15 and 16 will be examinedbelow. Since the optical system shown in FIG. 15 includes a singledetection system for a magneto-optical signal and a tracking servosignal, some rays in the radial direction are masked by the aperture 12,and the amplitude of the tracking signal is undesirably lowered. Thissuggests that rays which are masked to reduce crosstalk components froman adjacent track include components modulated by a groove grossingsignal. The relationship between the aperture ratio and the decrease inamplitude of the tracking signal is almost constant independently of thecombinations of optical heads and discs if d1/p remains the same. Whend1/p becomes large, the amplitude of the tracking error signal obtainedwhen no aperture is arranged becomes small. In addition, when theaperture ratio becomes small, the amplitude of the tracking error signaltends to decrease immediately (see FIG. 17).

Based on the above description, setting of an optimal aperture ratiocorresponding to d1/p will be examined below. From equation (23'), whenthe track pitch is small as compared to the 1/e² diameter of the lightspot, i.e., when d1/p is large, if the crosstalk amount from an adjacenttrack is to be decreased by a predetermined amount as compared to theamount obtained when no aperture is arranged, an aperture having asmaller aperture ratio must be inserted in the optical path. When thedisc substrate thickness error is not strictly managed, a largespherical aberration may be generated. For this reason, an aperturehaving a still smaller aperture ratio must be inserted in the opticalpath. Meanwhile, a decrease in amplitude of the tracking signal causedby insertion of the aperture poses a problem.

In the experiments using the combination of the first example (d1/p=0.9)with a disc substrate thickness error=80 μm (W40=0.44λ or equivalent),the aperture 12 preferably had B1/A1=0.50 to 0.85. More preferably, theaperture 12 has B1/A1=0.55 to 0.75. When such an aperture was insertedin the vicinity of the pupil of the light-receiving system, crosstalkcomponents from an adjacent track could be effectively reduced, and adecrease in amplitude of the tracking signal fell within an allowablerange.

When this condition is substituted in equation (23'), the constant Kpreferably satisfies:

    0.74<K<1.09

More preferably, the constant K satisfies:

    0.79<K<0.99

From these conditions, B1/A1 preferably satisfies:

    0.74-0.21·(d1/p)-0.26·W40.sup.2 <B1/A1<1.09-0.21·(d1/p)-0.26·W40.sup.2  (24)

More preferably, B1/A1 satisfies:

    0.79-0.21·(d1/p)-0.26·W40.sup.2 <B1/A1<0.99-0.21·(d1/p)-0.26·W40.sup.2  (25)

for W40≠0, 0<B1/A1<1

When the optical system described above with reference to FIGS. 19 and20 suffers a spherical aberration due to, e.g., a disc substratethickness error, the influence of crosstalk components from an adjacenttrack can be effectively reduced by inserting the aperture 12 given byequation (23'). A magneto-optical head optical system including theaperture 12 shown in FIGS. 19 and 20 will be examined below. The opticalsystem shown in FIG. 19 includes independent detection systems for amagneto-optical signal and servo signals. From equation (23'), when thetrack pitch is small as compared to the 1/e² diameter of the light spot,i.e., when d/p is large, if the crosstalk amount from an adjacent trackis to be decreased by a predetermined amount as compared to the amountobtained when no aperture is arranged, an aperture having a smalleraperture ratio must be inserted in the optical path.

When the disc substrate thickness error is not strictly managed, a largespherical aberration may be generated. For this reason, an aperturehaving a still smaller aperture ratio must be inserted in the opticalpath. Although the aperture 12 does not influence the servo signal inthis case, deterioration of the C/N (carrier to noise) ratio due to adecrease in carrier level caused by insertion of the aperture must betaken into consideration. The relationship between the aperture ratioand the carrier level is almost constant independently of thecombinations of optical heads and discs if d1/p remains the same. Whend1/p becomes large, the carrier level tends to drop. In addition, whenthe wavelength is shortened, the photoelectric conversion efficiency ofthe photodetector is lowered, and the carrier level tends to drop.

In the experiments using the combination of the first example(d1/p=0.9), the aperture 12 preferably had B1/A1=0.40 to 0.85. Morepreferably, the aperture 12 has B1/A1=0.45 to 0.75.

When this condition is substituted in equation (23'), the constant Kpreferably satisfies:

    0.64<K<1.09

More preferably, the constant K satisfies:

    0.69<K<0.99

From these conditions, B1/A1 preferably satisfies:

    0.64-0.21·(d1/p)-0.26·W40.sup.2 <B1/A1<1.09-0.21·(d1/p)-0.26·W40.sup.2  (26)

More preferably, B1/A1 satisfies:

    0.69-0.21·(d1/p)-0.26·W40.sup.2 <B1/A1<0.99-0.21·(d1/p)-0.26·W40.sup.2  (27)

for W40≠0, 0<B1/A1<1

Furthermore, as in the embodiment described above with reference toFIGS. 23 and 24, the influence of crosstalk components from an adjacenttrack can be effectively reduced by receiving a light beam reflected bythe disc using the photodetectors 7-1 and 7-2 each of which has alight-receiving portion given by equation (23') in an optical headoptical system which suffers a spherical aberration due to, e.g., a discsubstrate thickness error (see FIGS. 23 and 24).

Since the optical system shown in FIG. 23 includes independent detectionsystems for a magneto-optical signal and a tracking servo signal, thedimension, in the radial direction, of the light-receiving portion ofeach photodetector is represented by B1 in place of the aperture, andB1/A1 preferably satisfies inequality (26). More preferably, B1/A1satisfies inequality (27). When photodetectors with such dimensions areused, crosstalk components from an adjacent track can be effectivelyreduced.

Contributions of the coma and the spherical aberration to the light spotare independent, and an increase in crosstalk amount from an adjacenttrack is independently caused by the coma and the spherical aberration.Therefore, it is confirmed based on simulation and experimental resultsthat the total crosstalk amount upon simultaneous generation of the twoaberrations corresponds to a square mean of independently generatedcrosstalk amounts. Thus, setting of an optimal aperture ratiocorresponding to d1/p upon simultaneous generation of the twoaberrations will be examined below.

From equations (14') and (23'), since the optimal aperture ratio B1/A1corresponds to a square mean of the contribution amounts of a coma and aspherical aberration, it can be expressed by:

    B1/A1=K-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2 }                        (28)

When a single detection system is used for detecting a magneto-opticalsignal and a tracking servo signal, if a disc tilt and a disc substratethickness error are simultaneously generated, from inequalities (15) and(24), B1/A1 preferably satisfies:

    0.74-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2 }<B1/A1<1.09-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2}                         (29)

More preferably, from inequalities (16) and (25), B1/A1 satisfies:

    0.79-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2).sup.2 }<B1/A1<0.99-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2}                         (30)

For W31≠0, W40≠0, 0<B1/A1<1

For example, in the combination of the first example (d1/p=0.9), when adisc tilt=4 mrad. (W31=0.2λ or equivalent) and a disc substratethickness error=80 μm (W40=0.44λ or equivalent) are simultaneouslygenerated, the aperture 12 preferably has B1/A1=0.48 to 0.83. Morepreferably, the aperture has B1/A1=0.53 to 0.73.

When independent detection systems are used for detecting amagneto-optical signal and a tracking servo signal, if a disc tilt and adisc substrate thickness error are simultaneously generated, frominequalities (17) and (26), B1/A1 preferably satisfies:

    0.64-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2 }<B1/A1<1.09-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2 }                        (31)

More preferably, from inequalities (18) and (27), B1/A1 satisfies:

    0.69-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2).sup.2 }<B1/A1<0.99-0.21·(d1/p)-√ {(0.25·W31).sup.2 +(0.26·W40.sup.2).sup.2 }                        (32)

For W31≠0, W40≠0, 0<B1/A1<1

For example, in the combination of the first example (d1/p=0.9), when adisc tilt=4 mrad. (W31=0.2λ or equivalent) and a disc substratethickness error=80 μm (W40=0.44λ or equivalent) are simultaneouslygenerated, the aperture 12 preferably has B1/A1=0.38 to 0.83. Morepreferably, the aperture has B1/A1=0.43 to 0.73.

As described above, when an aperture for masking marginal rays, in theradial direction of returned light from a disc is arranged in a regionsufficiently separated from the focal plane of an optical headlight-receiving system, e.g., in the vicinity of the pupil of thelight-receiving system, the influence of crosstalk components from anadjacent track can be effectively reduced. An optimal ratio of theaperture width in the radial direction to the beam diameter of areturned light beam (aperture ratio B1/A1) can be set as follows.

(1) When coma and spherical aberration are satisfactorily corrected:

an aperture ratio determined by the ratio d/p of the light spot diameterd of the optical head in the radial direction to the track pitch p ofthe disc can be used.

(2) When coma is dominant:

an aperture ratio determined by the ratio d/p of the light spot diameterd of the optical head in the radial direction to the track pitch p ofthe disc, and the wave aberration coefficient W31 of the coma can beused.

(3) When spherical aberration is dominant:

an aperture ratio determined by the ratio d/p of the light spot diameterd of the optical head in the radial direction to the track pitch p ofthe disc, and the wave aberration coefficient W40 of the sphericalaberration can be used.

(4) When both coma and spherical aberration are simultaneouslygenerated:

an aperture ratio determined by the ratio d/p of the light spot diameterd of the optical head in the radial direction to the track pitch p ofthe disc, the wave aberration coefficient W31 of the coma, and the waveaberration coefficient W40 of the spherical aberration can be used.

Therefore, even when a disc suffers from a tilt or a substrate thicknesserror, crosstalk components from an adjacent track can be effectivelyreduced.

The same effect as described above can be expected by masking marginalrays, in the radial direction, of returned light by modifying the shapeof the photodetector. When the present invention is used, easyadjustment of the aperture or photodetector is greatly improved ascompared to the prior art in which a pinhole is inserted in the focalplane of the light-receiving system, and the arrangement of the presentinvention is not easily influenced by a change in temperature or aging.Since the aperture is inserted in the light-receiving system, aconsiderable decrease in light utilization efficiency due to theinsertion of an annular aperture can be prevented unlike in the priorart, and the arrangement of the present invention is suitable for, e.g.,a magneto-optical disc recording/reproduction apparatus which requireshigh power upon recording/erasing of information.

The arrangement according to the 10th embodiment of the presentinvention will be described below with reference to FIGS. 62 to 64. FIG.62 is a sectional view, in the track direction, of a magneto-opticaldisc 5 in an optical system of the present invention, FIG. 63 is asectional view, in the radial direction, of the disc 5, and FIG. 64 is aview showing the shape of an aperture 38. FIGS. 62 and 63 particularlyillustrate only an optical system for receiving returned light from adisc so as to explain the principle of the present invention.

Referring to FIGS. 62 and 63, an objective lens 4 forms a fine lightspot 9 on the information recording surface of the magneto-optical disc5. Assume that the light spot 9 is focused in an ideal state, and thecoma and the spherical aberration are satisfactorily corrected.

Referring to FIG. 62, an arrow 14 indicates the track direction, and aplurality of marks are aligned in the direction of the arrow 14. A inFIG. 62 represents the width of a light beam of the most marginalportion of an optical system for forming a light spot on a disc, andthis width A is determined by an aperture 4' of an objective lens. Thewidth A represents the NA of the light projection system of theobjective lens. FIG. 62 also illustrates, beside the light-receivingsystem, a state wherein a side lobe due to the influence of the aperture4' of the objective lens is generated in the light spot 9. The lightspot is illustrated in a state observed in the direction of rays, andits b-b' section corresponds to the track direction. The light intensitydistribution of the light spot in the b-b' section is the same as thatshown in FIG. 48, and a side lobe having an intensity as high as about2% of the central intensity is generated.

When information of a mark on a given track is reproduced in this state,the side lobe due to the influence of the aperture of the objective lensreproduces information of an adjacent mark on this track, and thereproduced information generates a distortion or shift in a reproductionsignal to be originally reproduced. This is called an intersymbolinteraction, and causes an increase in jitter. Such a problem of theintersymbol interaction is unavoidable in a conventional optical system,and seriously disturbs an increase in density.

Returned light from the magneto-optical disc 5 is collimated via anobjective lens 4, and is guided toward a photodetector 7 by a condenserlens 6. The aperture 38 is arranged between the objective lens 4 and thecondenser lens 6, i.e., in the vicinity of the pupil of thelight-receiving system, and masks marginal rays in the track direction,so that these rays do not reach the photodetector 7. B in FIG. 62represents the width of a light beam of the most marginal portion of thelight-receiving system, and the width B is determined by the aperture38. The width B represents the NA of the light-receiving system of theobjective lens.

FIG. 62 illustrates rays which pass various height positions of thepupil diameter, and of these rays, hatched rays which are masked by theaperture 38 form a side lobe in the light spot on the disc under theinfluence of the aperture 4' of the objective lens 4. Therefore, most ofthe information components reproduced from an adjacent mark on thesingle track are included in these marginal rays, and the intersymbolinteraction can be reduced by masking these rays. The side lobe due tothe influence of the aperture 4' and marginal rays in the pupil of thelight-receiving system normally have a good correspondence therebetweenalthough they do not have a strict one-to-one correspondencetherebetween unlike that between the light spot on the disc and thelight spot of the light-receiving system shown in FIG. 4.

Similarly, FIG. 63 illustrates, beside the light-receiving system, astate wherein a side lobe due to the influence of the aperture 4' of theobjective lens 4 is generated in the light spot 9. The a-a' section ofthe light spot corresponds to the radial direction. Returned light fromthe magneto-optical disc 5 is collimated via the objective lens 4, andis guided toward the photodetector 7 by the condenser lens 6. Theaperture 38 has a dimension in the radial direction larger than the beamdiameter A so as not to mask marginal rays in the radial direction. Thisis to guide modulated components of a tracking signal from the disc tothe photodetector as much as possible since these components arespatially distributed in the radial direction in the pupil of thelight-receiving system.

FIG. 64 is a front view of the aperture 38. The dimension, in the trackdirection, of the aperture is B2 (B2<A2), and the dimension, in theradial direction, of the aperture is C2 (C2>A2). If the objective lenshas a focal length fo=3 mm and NA=0.55, the beam diameter A2=3.3 mm. Aswill be described later, since B2 is selected to be about several mm,easy adjustment of the aperture is greatly improved as compared to theprior art in which a pinhole having a diameter of 15 μm is inserted inthe focal plane of the light-receiving system, and the aperture is noteasily influenced by a change in temperature or aging. Since theaperture 38 is inserted in the light-receiving system, a considerabledecrease in light utilization efficiency due to the insertion of anannular aperture can be prevented unlike in the prior art, and thisarrangement is suitable for, e.g., a magneto-optical discrecording/reproduction apparatus which requires high power uponrecording/erasing of information.

FIG. 65 shows the experimental results of a jitter reduction effectobtained using combinations of two optical heads (optical heads 1 and 2)and four discs (discs 1 to 4) in the optical system according to the10th embodiment of the present invention. Optical head 1 has awavelength λ=780 nm, and the NA=0.55 of the objective lens, and opticalhead 2 has a wavelength λ=680 nm, and the NA=0.60 of the objective lens.Disc 1 has a track pitch p=1.4 μm, and a shortest mark length=0.75 μm ofa carrier (reproduction signal), disc 2 has a track pitch p=1.1 μm, anda shortest mark length=0.64 μm of a carrier, disc 3 has a track pitchp=1.6 μm, and a shortest mark length=0.78 μm of a carrier, and disc 4has a track pitch p=0.8 μm, and a shortest mark length=0.47 μm of acarrier. Experimental conditions include: mark edge recording, a 1-7modulation method for symbols, and a linear velocity=15 m/s. The tiltand substrate thickness error of each disc are negligible.

FIG. 65 shows the calculation results for a combination of optical head1 and disc 1 as the first example (mark □ in FIG. 65). FIG. 65 shows thecalculation results for a combination of optical head 2 and disc 2 asthe second example (mark  in FIG. 65). FIG. 65 shows the calculationresults for a combination of optical head 1 and disc 2 as the thirdexample (mark ∘ in FIG. 65). FIG. 65 shows the calculation results for acombination of optical head 1 and disc 3 as the fourth example (mark Δin FIG. 65). FIG. 65 shows the calculation results for a combination ofoptical head 2 and disc 4 as the fifth example (mark × in FIG. 65). FIG.65 shows the calculation results for a combination of optical head 2 anddisc 1 as the sixth example (mark ▪ in FIG. 65). In FIG. 65, theabscissa represents the ratio of the width B2, in the track direction,of the aperture 38 to the beam diameter A2, and the ordinate representsthe jitter amount.

For example, in the combination of the first example, the reductioneffect is maximized when B2/A2 is set to be about 0.75, and is graduallyreduced when B2/A2 is set to be 0.75 or less. Although information froman adjacent mark is masked by the aperture 38, since the carrier levelis lowered as B2/A2 becomes smaller, the C/N ratio deteriorates and thejitter amount due to noise increases. As can be seen from the abovedescription, an aperture ratio B2/A2 which minimizes the jitter amountvaries depending on combinations of optical heads and discs.

In order to examine the relationship between the jitter reduction effectand the combinations of the optical heads and discs when the aperture isused, Table 7 below summarizes the light spot diameter determined by theNA of the objective lens and the wavelength of the semiconductor laserof the optical head, and the shortest mark length of the disc. Assumingthat the light spot diameter is symmetrical in the radial and trackdirections, Table 1 is quoted.

                  TABLE 7                                                         ______________________________________                                        (Spot Diameter and Shortest Mark Length)                                                   d2/m                                                                            Optical Head 1                                                                           Optical Head 2                                      Shortest Mark   = 780 nm   = 680 nm                                           Length (μm) NA = 0.55  NA = 0.60                                           ______________________________________                                        0.47                      2.11                                                0.64           1.97       1.55                                                0.75           1.68       1.32                                                0.78           1.62                                                           ______________________________________                                    

Table 7 shows the ratio d2/m of the 1/e² diameter d of the light spotshown in Table 1 and the shortest mark length m of the disc. As can beunderstood from FIG. 65 and Table 7, when the coma and the sphericalaberration are satisfactorily corrected, the aperture ratio (B2/A2) foreffectively reducing the jitter amount caused by, e.g., the intersymbolinteraction is an amount associated with the light spot diameterdetermined by the NA of the objective lens of the optical head and thewavelength of the semiconductor laser, and the shortest mark length ofthe disc. Therefore, it can be estimated from this fact that an optimalratio of the aperture width in the track direction to the beam diameterof the return light includes these parameters.

The aperture ratio (B2/A2) which can minimize the jitter amount in therespective calculation results will be examined below with reference tothe combinations of optical heads and discs. FIG. 66 shows theexamination results of the relationship between B2/A2 and d2/m. In FIG.66, the abscissa represents d2/m, and the ordinate represents theaperture ratio (B2/A2) which can minimize the jitter amount. As can beseen from FIG. 66, the combinations of the first to sixth examples aredistributed almost on a straight line. An approximation of the straightline is as follows:

    B2/A2=0.92-0.1·(d2/m)                             (33)

As is apparent from FIG. 65, an aperture ratio range with a relativelylow jitter amount of about ±0.15, which range has the minimum jitteramount given by equation (33) as the center, exists. When the apertureratio is selected within a range which satisfies the following relation,the jitter amount caused by, e.g, the intersymbol interaction can beminimized, and stable signal reproduction is allowed.

    0.77-0.1·(d2/m)<B2/A2<1.07-0.1·(d2/m)    (34)

More preferably, the aperture ratio range satisfies:

    0.82-0.1·(d2/m)<B2/A2<1.02-0.1·(d2/m)    (35)

When such an aperture is inserted in the vicinity of the pupil of thelight-receiving system, information reproduced from an adjacent mark ona single track can be masked, and the jitter amount caused by theintersymbol interaction can be reduced.

Note that the optical system described above with reference to FIGS. 39and 40 does not suffer a decrease in amplitude of a tracking signalwhich poses a problem in the optical system shown in FIG. 15 since raysin the radial direction are not masked although a single detectionsystem is used for detecting a magneto-optical signal and a trackingservo signal. In this embodiment, although some rays in the trackdirection are masked by the aperture 38, they do not largely influence aservo signal. Therefore, independently of whether a single detectionsignal is used or independent detection systems are used for detecting amagneto-optical signal and servo signals, the aperture ratio preferablysatisfies inequality (34) or (35).

FIG. 67 shows the experimental results of the jitter reduction effectobtained using the combinations of optical head 1 with disc 1 andoptical head 2 with disc 2 in the optical system of the embodimentdescribed above with reference to FIGS. 34 and 35. The experimentalconditions are the same as those in the 10th embodiment of the presentinvention. The disc tilts are 3.9 mrad. and 6.5 mrad. for thecombination of optical head 1 and disc 1, and are 2.6 mrad. and 4.4mrad. for the combination of optical head 2 and disc 2.

Depending on the difference in NA of the objective lens and λ of theoptical head, optical head 2 generates a coma about 1.5 times thatgenerated by optical head 1. From equation (11), in optical head 1, whenthe disc is tilted at 3.9 mrad., W31=0.188λ; when the disc is tilted at6.5 mrad., W31=0.313λ. In optical head 2, when the disc is tilted at 2.6mrad. and 4.4 mrad., the same comas as described above are generated.The abscissa represents the ratio of the width B2, in the trackdirection, of the aperture 38 to the beam diameter A2, and the ordinaterepresents the jitter amount.

For example, in the combination of optical head 1 and disc 1, when thedisc is not tilted (W31=0), the reduction effect is maximized when B2/A2is set to be about 0.75. However, when a disc tilt=6.5 mrad. isgenerated (W31=0.313λ), the reduction effect is maximized when B2/A2 isset to be about 0.71. Depending on the disc tilt, the aperture ratioB2/A2 which minimizes the jitter amounts gradually shifts toward asmaller value. Although not shown, in combinations of optical head 1with discs 2 and 3, and optical head 2 with discs 1 and 4, the same disctilts as described above were given, and the same experiments wereperformed.

In order to examine the relationship between the jitter reduction effectwhen an aperture is used in an optical disk optical system which suffersa coma due to, e.g., the disc tilt, and the combinations of opticalheads and discs, Table 8 summarizes the light spot diameter determinedby the NA of the objective lens of the optical head and the wavelengthof the semiconductor laser, and the shortest mark length of the disc.Assuming that the light spot diameter is symmetrical in the radial andtrack directions, Table 3 is quoted.

                  TABLE 8                                                         ______________________________________                                        (Spot Diameter and Track Pitch)                                                            d2/m                                                                            Optical Head 1                                                                           Optical Head 2                                      Shortest Mark   = 780 nm   = 680 nm                                           Length (μm) NA = 0.55  NA = 0.60                                           Disc Tilt (mrad.)                                                                            3.9    6.5     2.6   4.4                                       ______________________________________                                        0.47                          2.17  2.24                                      0.64           2.00   2.03    1.59  1.64                                      0.75           1.70   1.73    1.36  1.40                                      0.78           1.64   1.66                                                    ______________________________________                                    

Table 8 shows the ratio d2/m of the 1/e² diameter d of the light spotshown in Table 3 and the shortest mark length m of the disc. As can beunderstood from FIG. 67 and Table 8, when a coma is generated due to,e.g., a disc tilt, the aperture ratio (B2/A2) for effectively reducingthe jitter amount caused by, e.g., the intersymbol interaction is anamount associated with the light spot diameter determined by the NA ofthe objective lens of the optical head and the wavelength of thesemiconductor laser, the shortest mark length of the disc, and the comacoefficient W31. Therefore, it can be estimated from this fact that anoptimal ratio of the aperture width in the track direction to the beamdiameter of the return light includes these parameters.

The aperture ratio (B2/A2) which can minimize the jitter amount in therespective calculation results will be examined below with reference tothe combinations of optical heads and discs. FIG. 68 shows theexamination results of the relationship between B2/A2 and d2/m. In FIG.68, the abscissa represents d2/m, and the ordinate represents theaperture ratio (B2/A2) which can minimize the jitter amount. As can beseen from FIG. 68, the combinations of the first to sixth examples aredistributed almost on a straight line. An approximation of the straightline is as follows:

When W31=0 (when the disc is not tilted), from equation (31), we have:

    B2/A2=0.92-0.1·(d2/m)

When W31=0.188λ, we have:

    B2/A2=0.90-0.1·(d2/m)                             (36)

When W31=0.313λ, we have:

    B2/A2=0.88-0.1·(d2/m)                             (37)

As can be seen from these equations, when the coma increases, B2/A2 mustbe decreased in proportion to the coma coefficient so as to decrease thejitter amount caused by, e.g., the intersymbol interaction. Fromequations (33), (36), and (37), when W31 is introduced in theseequations, we have:

    B2/A2=0.92-0.1·(d2/m)-0.12·W31           (38)

Also, as is apparent from FIG. 67, an aperture ratio range with arelatively low jitter amount of about ±0.15, which range has the minimumjitter amount given by equation (33) as the center, exists. When theaperture ratio is selected within a range which satisfies the followingrelation, the jitter amount caused by, e.g, the intersymbol interactioncan be minimized, and stable signal reproduction is allowed.

    0.77-0.1·(d2/m)-0.12·W31<B2/A2<1.07-0.1·(d2/m)-0.12·W31                                           (39)

More preferably, the aperture ratio range satisfies:

    0.82-0.1·(d2/m)-0.12·W31<B2/A2<1.02-0.1·(d2/m)-0.12·W31                                           (40)

For W31≠0, 0<B2/A2<1

When such an aperture is inserted in the vicinity of the pupil of thelight-receiving system, information reproduced from the adjacent mark ismasked, and the jitter amount due to, e.g., the intersymbol interactioncan be reduced.

The arrangement according to the 11th embodiment of the presentinvention will be described below with reference to FIGS. 69 and 70.FIG. 69 is a sectional view, in the track direction, of amagneto-optical disc 5 in an optical system of the present invention,and FIG. 70 is a sectional view, in the radial direction, of the disc 5.FIGS. 69 and 70 particularly illustrate only the light-receiving systemfor explaining the principle of the present invention.

FIG. 69 illustrates, beside the light-receiving system, a state whereina side lobe due to a spherical aberration is generated in a light spot9. The spherical aberration is generated due to a manufacturing error ofan objective lens and a disc substrate thickness error, and has a sidelobe which is symmetrical about the axis of rotation. Although the lightspot shown in FIG. 62 has a side lobe which is symmetrical about theaxis of rotation, when a spherical aberration is generated, a side lobehaving a higher peak intensity is observed. The light spot isillustrated in a state observed in the direction of rays, and its b-b'section corresponds to the track direction. When information of a markon a given track is reproduced in this state, the side lobe due to thespherical aberration reproduces information of an adjacent mark on thesingle track, and the reproduced information generates a distortion orshift in a reproduction signal to be originally reproduced. This iscalled an intersymbol interaction, and causes an increase in jitter.When the NA of the objective lens is to be increased, the allowablemanufacturing error of the objective lens and the disc substratethickness error must be reduced, and a problem of the intersymbolinteraction from an adjacent mark due to the spherical aberrationseriously disturbs an increase in density.

Returned light from the magneto-optical disc 5 is collimated via anobjective lens 4, and is guided toward a photodetector 7 by a condenserlens 6. An aperture 38 is arranged between the objective lens 4 and thecondenser lens 6, i.e., in the vicinity of the pupil of thelight-receiving system, and masks marginal rays in the track direction,so that these rays do not reach the photodetector 7. FIG. 69 illustratesrays which pass various height positions of the pupil diameter, and ascan be seen from FIG. 69, of these rays, hatched rays which are maskedby the aperture 38 mainly form a side lobe in the light spot on thedisc. Therefore, most of the information components reproduced from anadjacent mark on a single track are included in these marginal rays, andthe intersymbol interaction can be reduced by masking these rays. Theside lobe due to the spherical aberration and marginal rays in the pupilof the light-receiving system normally have a good correspondencetherebetween although they do not have a strict one-to-onecorrespondence therebetween unlike that between the light spot on thedisc and the light spot of the light-receiving system shown in FIG. 4.

In FIG. 70, an arrow 13 and the a-a' section of the light spotcorrespond to the radial direction. Returned light from themagneto-optical disc 5 is collimated via the objective lens 4, and isguided toward the photodetector 7 by the condenser lens 6. The aperture38 has a dimension in the radial direction larger than the beam diameterA so as not to mask marginal rays in the radial direction. This is toguide modulated components of a tracking signal from the disc to thephotodetector as much as possible since these components are spatiallydistributed in the radial direction in the pupil of the light-receivingsystem.

FIG. 71 shows the experimental results of the jitter reduction effectusing combinations of optical head 1 with disc 2 and optical head 2 withdisc 2 in the optical system according to the 11th embodiment of thepresent invention. The experimental conditions are the same as those inthe 10th embodiment of the present invention. The substrate thicknesserrors of the disc are ±50 μm, ±75 μm, and ±100 μm for the combinationof optical head 1 and disc 1, and are ±31 μm, ±46 μm, and ±62 μm for thecombination of optical head 2 and disc 2. Depending on the differencesin NA of the objective lens and in wavelength, optical head 2 generatesa spherical aberration about 1.6 times that generated by optical head 1.In equation (19), when optical head 1 suffers substrate thickness errorsof ±50 μm, ±75 μm, and ±100 μm, W40=0.28λ, 0.41λ, and 0.55λ,respectively. On the other hand, when optical head 2 suffers substratethickness errors of ±31 μm, ±46 μm, and ±62 μm, equivalent sphericalaberrations are generated. The abscissa represents the ratio of thewidth B2, in the track direction, of the aperture 38 to the beamdiameter A2, and the ordinate represents the jitter amount.

For example, in the combination of optical head 1 and disc 1, when thedisc suffers no substrate thickness error (W40=0), the reduction effectis maximized when B2/A2 is set to be about 0.75. However, when a discsubstrate error=±100 μm is generated (W40=0.55λ), the reduction effectis maximized when B2/A2 is set to be about 0.715. Depending on thesubstrate thickness error, the aperture ratio B2/A2 which minimizes thejitter amounts gradually shifts toward a smaller value. Although notshown, in combinations of optical head 1 with discs 2 and 3, and opticalhead 2 with discs 1 and 4, the same substrate thickness errors asdescribed above were given, and the same experiments were performed.

In order to examine the relationship between the jitter reduction effectwhen an aperture is used in an optical disk optical system which suffersa spherical aberration due to, e.g., the substrate thickness error, andthe combinations of optical heads and discs, Table 9 summarizes thelight spot diameter determined by the NA of the objective lens of theoptical head and the wavelength of the semiconductor laser, and theshortest mark length of the disc. Assuming that the light spot diameteris symmetrical in the radial and track directions, Table 5 is quoted.

                  TABLE 9                                                         ______________________________________                                        (Spot Diameter and Track Pitch)                                               ______________________________________                                                d2/m                                                                            Optical Head 1  Optical Head 2                                      Shortest Mark                                                                            = 780 nm        = 680 nm                                           Length (μm)                                                                          NA = 0.55       NA = 0.60                                           ______________________________________                                        Substrate 50     75       100   31    46   62                                 Thickness Error                                                               (μm)                                                                       0.47                             2.11 2.11 2.13                               0.64      1.97   1.97     1.98  1.55  1.55 1.56                               0.75      1.68   1.68     1.69  1.32  1.32 1.33                               0.78      1.62   1.62     1.63                                                ______________________________________                                    

Table 9 shows the ratio d2/m of the 1/e² diameter d of the light spotshown in Table 5 and the shortest mark length m of the disc. As can beunderstood from FIG. 71 and Table 9, when a spherical aberration iscaused by a substrate thickness error, the aperture ratio (B2/A2) foreffectively reducing the jitter amount caused by, e.g., the intersymbolinteraction is an amount associated with the light spot diameterdetermined by the NA of the objective lens of the optical head and thewavelength of the semiconductor laser, the shortest mark length of thedisc, and the spherical aberration coefficient W40. Therefore, it can beestimated from this fact that an optimal ratio of the opening width inthe track direction to the beam diameter of the return light includesthese parameters.

The aperture ratio (B2/A2) which can minimize the jitter amount in therespective calculation results will be examined below with reference tothe combinations of optical heads and discs. FIG. 72 shows theexamination results of the relationship between B2/A2 and d2/m. In FIG.72, the abscissa represents d2/m, and the ordinate represents theaperture ratio (B2/A2) which can minimize the jitter amount. As can beseen from FIG. 72, the combinations of the first to sixth examples aredistributed almost on a straight line. An approximation of the straightline is as follows:

When W40=0 (when the disc suffers no substrate thickness error), fromequation (33), we have:

    B2/A2=0.92-0.1·(d2/m)

When W40=0.28λ, we have:

    B2/A2=0.91-0.1·(d2/m)                             (41)

When W40=0.41λ, we have:

    B2/A2=0.90-0.1·(d2/m)                             (42)

When W40=0.55λ, we have:

    B2/A2=0.88-0.1·(d2/m)                             (43)

As can be seen from these equations, when the spherical aberrationincreases, B2/A2 must be decreased in proportion to a square of thespherical aberration coefficient so as to decrease the jitter amountcaused by, e.g., the intersymbol interaction. From equations (33), and(41) to (43), when W40 is introduced in these equations, we have:

    B2/A2=0.92-0.19·(d2/m)-0.12·W40.sup.2    (44)

Also, as is apparent from FIG. 71, an aperture ratio range with arelatively low jitter amount of about ±0.15, which range has the minimumjitter amount given by equation (44) as the center, exists. When theaperture ratio is selected within a range which satisfies the followingrelation, the jitter amount caused by, e.g, the intersymbol interactioncan be minimized, and stable signal reproduction is allowed.

    0.77-0.1·(d2/m)-0.12·W40.sup.2 <B2/A2<1.07-0.1·(d2/m)-0.12·W40.sup.2   (45)

More preferably, the aperture ratio range satisfies:

    0.82-0.1·(d2/m)-0.12·W40.sup.2 <B2/A2<1.02-0.1·(d2/m)-0.12·W40.sup.2   (46)

For W40≠0, 0<B2/A2<1

When such an aperture is inserted in the vicinity of the pupil of thelight-receiving system, information reproduced from an adjacent mark ismasked, and the jitter amount due to, e.g., the intersymbol interactioncan be reduced.

Contributions of the coma and the spherical aberration to the light spotare independent, and an increase in jitter amount due to the intersymbolinteraction is independently caused by the coma and the sphericalaberration. Therefore, it is confirmed based on simulation andexperimental results that the total jitter amount upon simultaneousgeneration of the two aberrations corresponds to a square mean ofindependently generated crosstalk amounts. Thus, setting of an optimalaperture ratio corresponding to d2/m upon simultaneous generation of thetwo aberrations will be examined below.

From equations (38) and (44), since the optimal aperture ratio B2/A2corresponds to a square mean of the contribution amounts of a coma and aspherical aberration, it can be expressed by:

    B2/A2=0.92-0.1·(d2/m)-√ {(0.12·W31).sup.2 +(0.12·W40.sup.2).sup.2 }                        (47)

When a disc tilt and a disc substrate thickness error are simultaneouslygenerated, from inequalities (39) and (45), B2/A2 preferably satisfies:

    0.77-0.1·(d2/m)-√ {(0.12·W31).sup.2 +(0.12·W40.sup.2).sup.2 }<B2/A2<1.07-0.1·(d2/m)-√ {(0.12·W31).sup.2 +(0.12·W40.sup.2).sup.2 }(48)

More preferably, from inequalities (40) and (46), B2/A2 satisfies:

    0.82-0.1·(d2/m)-√ {(0.12·W31).sup.2 +(0.12·W40.sup.2).sup.2 }<B2/A2<1.02-0.1·(d2/m)-√ {(0.12·W31).sup.2 +(0.12·W40.sup.2).sup.2 }(49)

For W31≠0, W40≠0, 0<B2/A2<1

For example, in the combination of the first example (d2/m=1.68), when adisc tilt=4 mrad. (W31=0.2λ or equivalent) and the disc substratethickness error=80 μm (W40=0.44λ or equivalent) are simultaneouslygenerated, the aperture 12 preferably has B2/A2=0.57 to 0.87. Morepreferably, the aperture has B2/A2=0.62 to 0.82.

As described above, when marginal rays in the track direction of thelight-receiving system are masked, information reproduced from anadjacent mark can be masked, and the jitter amount caused by theintersymbol interaction can be reduced. As in the above embodiments inwhich crosstalk components from an adjacent track are reduced by maskingmarginal rays in the radial direction, means for masking marginal raysin the track direction may be arranged in the vicinity of the pupil ofthe light-receiving system or may be arranged in the far field regionsufficiently separated from the focal point of the light-receivingsystem. An optimal ratio of the aperture width in the track direction tothe beam diameter of a returned light beam (aperture ratio B2/A2) can beset as follows.

(1) When coma and spherical aberration are satisfactorily corrected:

an aperture ratio determined by the ratio d/m of the light spot diameterd of the optical head in the track direction to the shortest mark lengthrecorded on the disc can be used.

(2) When coma is dominant:

an aperture ratio determined by the ratio d/m of the light spot diameterd of the optical head in the track direction to the shortest mark lengthrecorded on the disc, and the wave aberration coefficient W31 of thecoma can be used.

(3) When spherical aberration is dominant:

an aperture ratio determined by the ratio d/m of the light spot diameterd of the optical head in the track direction to the shortest mark lengthrecorded on the disc, and the wave aberration coefficient W40 of thespherical aberration can be used.

(4) When both coma and spherical aberration are simultaneouslygenerated:

an aperture ratio determined by the ratio d/m of the light spot diameterd of the optical head in the track direction to the shortest mark lengthrecorded on the disc, the wave aberration coefficient W31 of the coma,and the wave aberration coefficient W40 of the spherical aberration canbe used.

Therefore, even when a disc suffers from a tilt or a substrate thicknesserror, information reproduced from an adjacent mark can be masked, andthe jitter amount due to the intersymbol interaction can be reduced. Theaperture 38 may have either a rectangular shape or an elliptical shape.Marginal rays in the track direction may be prevented from beingreceived by modifying the shape of the photodetector. These arrangementsmay be applied to either an optical system which includes a singledetection system for a magneto-optical signal and servo signals, or anoptical system which includes independent detection systems therefor.The reduction of jitter components caused by a coma due to a disc tiltor a spherical aberration due to a substrate thickness error has beenparticularly exemplified. The present invention is also effective whenan astigmatism or a defocus is generated.

When the present invention is used, easy adjustment of the aperture orphotodetector is greatly improved as compared to the prior art in whicha pinhole is inserted in the focal plane of the light-receiving system,and the arrangement of the present invention is not easily influenced bya change in temperature or aging. Since the aperture is inserted in thelight-receiving system, a considerable decrease in light utilizationefficiency due to the insertion of an annular aperture can be preventedunlike in the prior art, and the arrangement of the present invention issuitable for, e.g., a magneto-optical disc recording/reproductionapparatus which requires high power upon recording/erasing ofinformation.

As has been described above, when an aperture for masking marginal rays,in the radial direction, of returned light from a disc is arranged in aregion sufficiently separated from the focal plane of the optical headlight-receiving system, i.e., in the vicinity of the pupil of thelight-receiving system, the influence of crosstalk components from anadjacent track can be effectively reduced. The same effect as describedabove can be expected by masking marginal rays, in the radial direction,of returned light by modifying the shape of the photodetector.

As has been described above, when an aperture for masking marginal rays,in the track direction, of returned light from a disc is arranged in aregion sufficiently separated from the focal plane of the optical headlight-receiving system, i.e., in the vicinity of the pupil of thelight-receiving system, information reproduced from an adjacent mark canbe masked, and jitter components caused by the intersymbol interactioncan be reduced. The same effect as described above can be expected bymasking marginal rays, in the track direction, of returned light bymodifying the shape of the photodetector.

The present invention can be widely applied to either an optical systemwhich includes a single detection system for a magneto-optical signaland servo signals, or an optical system which includes independentdetection systems therefor. When the present invention is used, easyadjustment of the aperture or photodetector is greatly improved ascompared to the prior art in which a pinhole is inserted in the focalplane of the light-receiving system, and the arrangement of the presentinvention is not easily influenced by a change in temperature or aging.Since the aperture is inserted in the light-receiving system, aconsiderable decrease in light utilization efficiency due to theinsertion of an annular aperture can be prevented unlike in the priorart, and the arrangement of the present invention is suitable for, e.g.,a magneto-optical disc recording/reproduction apparatus which requireshigh power upon recording/erasing of information.

According to the present invention, crosstalk components from anadjacent track and intersymbol interaction factors from an adjacent markin the track direction due to a disc tilt can be effectively removed,and the allowable amount of the disc tilt is lessened, thus reducingcost of the disc. Also, crosstalk components from an adjacent track andintersymbol interaction factors from an adjacent mark in the trackdirection due to a disc substrate thickness error can be effectivelyremoved, and the allowable amount of the substrate thickness error islessened, thus reducing cost of the disc.

Since crosstalk components from an adjacent track or intersymbolinteraction factors from an adjacent mark in the track direction, whichare included in a side lobe portion, can be effectively removed byinserting a simple aperture or modifying the shape of the photodetector,the optical head can be rendered compact, and requirements as to thetilt adjustment precision, performance, and the like of the objectivelens are rendered less strict, thus reducing cost.

What is claimed is:
 1. An optical recording/reproducing apparatuscomprising:an irradiation optical system for irradiating a light beamfrom a light source onto a predetermined track of an optical recordingmedium having a plurality of neighboring tracks as a micro-light spot soas to effect recording/reproduction of information or reproduction ofinformation; a detection optical system for detecting a returned lightbeam from the optical recording medium, said detection optical systemincluding a first photodetector for detecting a reproduction signal ofinformation and a second photodetector for detecting a position signalof said light spot in a direction perpendicular to the track on asurface of the optical recording medium, and mask means, arranged in afar field region sufficiently separated from a focal plane of saiddetection optical system in an optical path of said detection opticalsystem, for masking marginal rays, in the direction perpendicular to thetrack, of the returned light beam to be incident on said first andsecond photodetectors, wherein a dominant optical aberration of thelight spot is a coma, and said mask means satisfies the followingcondition

    0.74-0.21·(d/p)-0.25·W31<B/A<1.09-0.21·(d/p)-0.25·W31, for 0<B/A<1

where A: is the beam diameter of the returned light beam in thedirection perpendicular to the track, B: is the aperture width of saidmask means for masking the returned light beam in the directionperpendicular to the track, d: is the 1/e² diameter of the light spot onthe optical recording medium, p: is the track pitch of the opticalrecording medium, and W31: is the wave aberration coefficient of thecoma, wherein information reproduced by a side lobe of the light spotfrom a track adjacent to the predetermined track is reduced uponreproduction of information of the predetermined track by the lightspot.
 2. An apparatus according to claim 1, wherein said mask meanscomprises an aperture smaller than a beam diameter, in the directionperpendicular to the track, of the returned light beam.
 3. An apparatusaccording to claim 1, wherein said mask means comprises a photodetectorwhich has a light-receiving portion smaller than a beam diameter, in thedirection perpendicular to the track, of the returned light beam.
 4. Anapparatus according to claim 1, wherein said mask means is arranged inthe vicinity of a pupil of said detection optical system.
 5. Anapparatus according to claim 1, wherein said mask means is arranged inan optical path in which the returned light beam from the opticalrecording medium is collimated.
 6. An apparatus according to claim 1,wherein said mask means further satisfies the following condition:

    0.79-0.21·(d/p)-0.25·W31<B/A<0.99-0.21·(d/p)-0.25·W31, for 0<B/A<1.


7. An optical recording/reproducing apparatus comprising:an irradiationoptical system for irradiating a light beam from a light source onto apredetermined track of an optical recording medium having a plurality ofneighboring tracks as a micro-light spot so as to performrecording/reproduction of information or reproduction of information; adetection optical system for detecting a returned light beam from theoptical recording medium, said detection optical system including afirst photodetector for detecting a reproduction signal of informationand a second photodetector for detecting a position signal of said lightspot in a direction perpendicular to the track on a surface of theoptical recording medium, and mask means, arranged in a far field regionsufficiently separated from a focal plane of said detection opticalsystem in an optical path of said detection optical system, for maskingmarginal rays, in the direction perpendicular to the track, of thereturned light beam to be incident on said first photodetector, whereina dominant optical aberration of the light spot is a coma, and said maskmeans satisfies the following condition:

    0.64-0.21·(d/p)-0.25·W31<B/A<1.09-0.21·(d/p)-0.25·W31, for 0<B/A<1

where A: is the beam diameter of the returned light beam in thedirection perpendicular to the track, B: is the aperture width of saidmask means for masking the returned light beam in the directionperpendicular to the track, d: is the 1/e² diameter of the light spot onthe optical recording medium, p: is the track pitch of the opticalrecording medium, and W31: is the wave aberration coefficient of thecoma,wherein information reproduced by a side lobe of the light spotfrom a track adjacent to the predetermined track is reduced uponreproduction of information of the predetermined track by the lightspot.
 8. An apparatus according to claim 7, wherein said mask meanscomprises an aperture smaller than a beam diameter, in the directionperpendicular to the track, of the returned light beam.
 9. An apparatusaccording to claim 7, wherein said mask means comprises a photodetectorwhich has a light-receiving portion smaller than a beam diameter, in thedirection perpendicular to the track, of the returned light beam.
 10. Anapparatus according to claim 7, wherein said mask means is arranged inthe vicinity of a pupil of said detection optical system.
 11. Anapparatus according to claim 7, wherein said mask means is arranged inan optical path in which the returned light beam from the opticalrecording medium is collimated.
 12. An apparatus according to claim 7,wherein said mask means further satisfies the following condition:

    0.69-0.21·(d/p)-0.25·W31<B/A<0.99-0.21·(d/p)-0.25·W31, for 0<B/A<1.


13. 13. An optical recording/reproducing apparatus comprising:anirradiation optical system for irradiating a light beam from a lightsource onto a predetermined track of an optical recording medium havinga plurality of neighboring tracks as a micro-light spot so as to performrecording/reproduction of information or reproduction of information; adetection optical system for detecting a returned light beam from theoptical recording medium, said detection optical system including afirst photodetector for detecting a reproduction signal of informationand a second photodetector for detecting a position signal of the lightspot in a direction perpendicular to the track on a surface of theoptical recording medium; and mask means, arranged in a far field regionsufficiently separated from a focal plane of said detection opticalsystem in an optical path of said detection optical system, for maskingmarginal rays, in the direction perpendicular to the track, of thereturned light beam to be incident on said first and secondphotodetectors, wherein a dominant optical aberration of the light spotis a spherical aberration, and said mask means satisfies the followingcondition:

    0.74-0.21·(d/p)-0.26·(W40).sup.2 <B/A<1.09-0.21·(d/p)-0.26·(W40).sup.2, for 0<B/A<1

where A: is the beam diameter of the returned light beam in thedirection perpendicular to the track, B: is the aperture width of saidmask means for masking the returned light beam in the directionperpendicular to the track, d: is the 1/e² diameter of the light spot onthe optical recording medium, p: is the track pitch of the opticalrecording medium, and W40: is the wave aberration coefficient of thespherical aberration,wherein information reproduced by a side lobe ofthe light spot from a track adjacent to the predetermined track isreduced upon reproduction of information of the predetermined track bythe light spot.
 14. An apparatus according to claim 13, wherein saidmask means comprises an aperture smaller than a beam diameter, in thedirection perpendicular to the track, of the returned light beam.
 15. Anapparatus according to claim 13, wherein said mask means comprises aphotodetector which has a light-receiving portion smaller than a beamdiameter, in the direction perpendicular to the track, of the returnedlight beam.
 16. An apparatus according to claim 13, wherein said maskmeans is arranged in the vicinity of a pupil of said detection opticalsystem.
 17. An apparatus according to claim 13, wherein said mask meansis arranged in an optical path in which the returned light beam from theoptical recording medium is collimated.
 18. An apparatus according toclaim 13, wherein said mask means further satisfies the followingcondition:

    0.79-0.21·(d/p)-0.26·(W40).sup.2 <B/A<0.99-0.21·(d/p)-0.26·(W40).sup.2, for 0<B/A<1.


19. An optical recording/reproducing apparatus comprising:an irradiationoptical system for irradiating a light beam from a light source onto apredetermined track of an optical recording medium having a plurality ofneighboring tracks as a micro-light spot so as to performrecording/reproduction of information or reproduction of information; adetection optical system for detecting a returned light beam from theoptical recording medium, said detection optical system including afirst photodetector for detecting a reproduction signal of informationand a second photodetector for detecting a position signal of the lightspot in a direction perpendicular to the track on a surface of theoptical recording medium; and mask means, arranged in a far field regionsufficiently separated from a focal plane of said detection opticalsystem in an optical path of said detection optical system, for maskingmarginal rays, in the direction perpendicular to the track, of thereturned light beam to be incident on said first photodetector, whereina dominant optical aberration of the light spot is a sphericalaberration, and said mask means satisfies the following condition:

    0.64-0.21·(d/p)-0.26·(W40).sup.2 <B/A<1.09-0.21·(d/p)-0.26·(W40).sup.2, for 0<B/A<1

where A: is the beam diameter of the returned light beam in thedirection perpendicular to the track, B: is the aperture width of saidmask means for masking the returned light beam in the directionperpendicular to the track, d: is the 1/e² diameter of the light spot onthe optical recording medium, p: is the track pitch of the opticalrecording medium, and W40: is the wave aberration coefficient of thespherical aberration,wherein information reproduced by a side lobe ofthe light spot from a track adjacent to the predetermined track isreduced upon reproduction of information of the predetermined track bythe light spot.
 20. An apparatus according to claim 19, wherein saidmask means comprises an aperture smaller than a beam diameter, in thedirection perpendicular to the track, of the returned light beam.
 21. Anapparatus according to claim 19, wherein said mask means comprises aphotodetector which has a light-receiving portion smaller than a beamdiameter, in the direction perpendicular to the track, of the returnedlight beam.
 22. An apparatus according to claim 19, wherein said maskmeans is arranged in the vicinity of a pupil of said detection opticalsystem.
 23. An apparatus according to claim 19, wherein said mask meansis arranged in an optical path in which the returned light beam from theoptical recording medium is collimated.
 24. An apparatus according toclaim 19, wherein said mask means further satisfies the followingcondition:

    0.69-0.21·(d/p)-0.26·(W40).sup.2 <B/A<0.99-0.21·(d/p)-0.26·(W40).sup.2, for 0<B/A<1.